So now that I have a decent weapon (10mm Pistol), I've decided to go and help Torr protect the brahmin from the 'bugmen'.

I've tried this a few times and I get the same message each time, "You failed to help Torr protect all the brahmin."

Only the first Radscorpion gets anywhere close, since it spawns next to you. The other ones are just chilling well away from Torr and the brahmin. I go and kill them all and it even tells me that I've successfully killed them all, and Torr even agrees, and yet I get this failure message when I leave.

What gives?

What you've described is the way to solve this quest. Are you using any mods that may affect gameplay?

You could also try talking to the Duntons lurking nearby, agree to their plan, and then convince them to call it off. That is an alternative, however. Defeating all the radscorpions should work.

In my opinion, the mysterious "bug men" are the Dunton brothers of course, and the radscorpions are there just to avert your attention (of course, you kill those anyway, for sport).

There several ways to solve this, but to my knowledge, they all involve dealing with Duntons in one way or another, up and including killing them then and there, if you so choice.

you have any patches, mods?

well, you'd better install at least killap's patch

No doubt about that.
Anyone looking for a smooth experience should get killap's patch, it gets rid of hundreds of bugs Black Isle Studios couldn't fix.

http://www.patches-scrolls.de/patch/1595/7/62759/download

No need to give links to some "strange" sites while there is official Killap's website:
http://www.killap.net/

For playing Fo2 these days Killap's patch is "a must". IMHO Restoration Project is good for first gameplay too - it's not altering gameplay, it's just restoring content made by original developers but not finished back then. There are no radical changes in story/gameplay.

I needed, though.
I once downloaded a file from the official website, in a formatted machine, and it was infected.
It certainly wasn't on purpose, but since then, I avoided downloading things there.
About the "strange" website, heh, I never had any issues with it.

Patches scrolls isn't "strange", it's been around for years and has patches for many games - including fan-made patches such as Killaps.

In theory killap's site should be the most up-to-date, but development on that patch has pretty much ended now AFAIK.

If it was "infected", it could've been a mitm... or a false positive. You can always re-check at virustotal.