One thing occured to me:

Wizardry Gaiden 4 feels more like a WRPG than a JRPG. Specifically:
* At the start of the game, there a 4 dungeons you can explore. One is completely optional (and isn't fully explorable at this point), but the other 3, which are towers, can be accessed in any order.
* The towers are rather substantial dungeons, and are filled with various traps and puzzles. Sometimes, puzzles have multiples solutions, which isn't something you would expect to see in a JRPG.
* There are NPCs, and NPC interaction similar to Wizardry 5 (though without the keyword dialog system due to the lack of a keyboard). You can do all sorts of things, like talking to them, giving them items, casting the game's Charm equivalent (which in at least one case allowed me to get a key from an NPC, so it's not useless), casting the game's Mindread equivalent (yes, such a thing exists), and even fighting them to the death. Unfortunately, this mechanic disappears once you've cleared all three towers, but it's definitely there for a good portion of the game.
* You can bypass many of the game's puzzles by having a thief-type pick the lock on a gate. It doesn't always work (and you may need to leave the dungeon to try again; I don't know the exact rule), and a few gates can't be picked this way, but it still provides a way to use one of your character's skills to bypass puzzles.

So, that might be an interesting game to look into. It's not perfect, as the combat difficulty stagnates once your level is high enough to tackle the top floor of any of the towers, but it's certainly an interesting game looking into (though you might want the translation patch if you want to enjoy the dialog and aren't able to read Japanese well); just don't cast the spell that's supposed to remove cursed items, as that particular spell will corrupt your save file.

My question is actually specifically wondering if you can get 2 Vi monsters to fight *without* modifying the game or save files in any way.

(Also, still wondering if you can smuggle Vi into your party for the final battle by entering the Cosmic Circle with her dead then reviving her, and hence get the evil ending with her help.)

By the way, once you defeat the Dark Savant for one of the better endings, each character in your party will say something; does Vi have any dialog in case you somehow beat the game with her in the party?

The answer to your 1st question depends on the answer to the 2nd, which i do not know.

The only scenario where i could see 2 Vis without changing stuff is in the showdown at the cosmic circle,
but only in case the game does spawn the Vi ID#441 if the RPC is dead in your party,
and also fails to remove the corpse.
In that case you could paralyze VI ID#441, kill Bela to ensure you can kite the DS away from Vi,
run far enough/stay out of view to end combat,
at that moment he must not have line of sight when you go back to the book and pen,
if Vi ID#441 is still paralyzed combat will not start and you can release Vi from party next to the ID#441,
that should(?) turn the RPC Vi into monster ID#395, now wait til the paralysis wears of and stand back to watch the battle.
Or initiate combat to give the lv8 Vi a headstart vs the lvl 17 Vi ;)

The RPC Vi does not have a line for winning the game, but has a line if the Dark Savant is killed with her present in party.
It is obsolete if we assume that it was not intended that players kill one of the other versions of the Dark savant
(the dark savant in the swamp, at ascension peak and in the CC arena are all different monsters/NPCs)
However the speech was probably recorded long before the final release,
and actors most likely spoke the full set of all possible messages that could be expected at that development stage,
so its not unusual to have messages as audio files with reference in scripts although they are not used in game.

That could also happen with turncoat, right?

In fact, if you turncoat all the enemies, you can get combat to end while the enemies are still alive. I believe you can even talk to the turncoated enemy if it's an NPC you could talk with if non-hostile, but does Vi ID#441 act like an NPC in this regard?

Is there a way to kill the Dark Savant in the swamp or Ascension Peak?

Also, there's that scene in the monastery where two NPCs talk to you in sequence and then disappear. Is it possible to kill either of them?

Then, of course, there's the NPC encounters with things like computers, that you can't normally attack. (For example, the computer that needs the black box to analyze data.) Is there a way to attack them? Do they even have stats?

Major Spoilers !

The Dark Savant in the Swamp, at Ascension Peak and his 1st instance at the Cosmic Circle
(which talks to Aletheides in the arena and kills him) have the flag "cannot be fought with" set.
Same for Bela at Mountain Wilderness, Aletheides, all Rapax involved in the Al Sedexus quest except Al-Adryian,
the scripted Lava Lord instance which walks from the lava lake to the cell with Rapax Rafe, the Screg hologram,
Phoonzang (need to select the related ending at CC) and last not least the Rapax Prince in the Castle and Camp.

But "cannot be fought with" is different to "no collision",
the party and other monsters just can not select these as target but strayed attacks can hit and cause dmg incl associated side effects.

Some examples:
A) the prince who is my favorite victim for the procedure:
In the Castle or the Camp, approach the door/curtain from the side so that the prince cant see the party when you open it,
else his NPC script is instantly triggered which causes him to fade out in view of the party.
Have ranged weapons equipped, preferably with either poison or kill effect assigned
(best use a ninja with death stars and medium hit chance, e.g. class change a caster;)
Select to enter combat mode, preferably turn based, move as close to the prince as possible,
best so that he covers all other concubines except one, and the latter to 99%.
To do so use they keyb to move tiny steps until the desired target is covered,
(ranged weapon attack symbol turns red or PC switches to another target)
then move 1cm back and target it again
You may eventually run out of concubines before the real job is done, therefore you need elemantals,
the canned one is fine as it will appear regardless of ceiling height/avail_space,
best have the spell and a ton of mana restauratives to not run out of summons.
In case only the cannot_be_fought_with creature is present, (i'll get to the in next example;)
a summoned creature must be turned hostile in the same turn as it appears,
else it will vanish.
This can be done with any cone type spell if it goes off after the summon.
To ensure that, either have a character with snakespeed ~30+ to summon/use the item,
and another to cast e.g. itching skin in the right direction.
Optionally select to move which triggers fixed initiative order, (exept for very fast PCs)
(either you added the PCs in appropriate order,or bring a dozen+ of the canned version)

B) Some of those monsters do not have potential victims near when they appear,
and not even random spawns moving near, e.g. the Dark Savant in the Swamp,
but iirc all disappear via script, and the disappearance is somewhat delayed, estimated .5 sec.
Therefore you can instantly (quick)save the game when you meet them
(for the Savant in the swamp ensure to wait til he dropped the fake AD, else you can not end the game!)
The save will have the script flagged as "done" but the cannot_be_fought_with guy is also still present,
and will stay until you relieve him of his misery.
Either wait til random spawns are near and kite them to the place, or use elementals like in example A.
Crock is near and sells/recharges mana stones (use only 2 charges!)

Note that killing the Screg hologram will also prevent to end the game without editing data.

In the Monasters, Aletheides can be killed without consequences,
but i usually ignore him(hug the walls) and let him stand there forever :)
The savant henchman is spawned by Aletheides's script,
so if quicksave works for the henchman (never tried, i only know how the scripts work;) its either the 1st or 2nd, never both.

The computers, spaceship etc have "lifeless" aka "no monster representative required/present in game" flagged, so no stats,
and as my old friend Sophokles already said: "you can kill a man but you cant kill an idea"
For the record the other NPC flags are: "trader", "recruitable", "no active interface" "cant talk to".