It finally worked, no thanks to GOG unfortunately.
I'd appreciate it, if GOG would upload a fix.
Simple: I switched to Linux at the end of XP support, like many others.
The next big amount of people will switch when Win7 support ends and then there'll be way more people around with your setup.

Thanks again to all community members involved in fixing it, especially to Skysect who noticed where the problem lay in the first place and who also provided a working fix.

The dummy dlls obviously didn't work as a fix.

Because most programs still run with WinXP and it needs very less resources.

But that's not the main problem here. For the offline installer you don't need the online stuff from galaxy. Every change is a possible troublemaker, not only for older operating systems like WinXP, but especially for helpful porting tools like Wine and also newer operating systems in the future.

Keep all this extra stuff for Galaxy out of the installer and only downloadable/patchable via Galaxy. Focus the offline installers to make the games playable and runable, nothing more. For many like me they are for perservation, including the time GOG and Galaxy are maybe gone.

The saddest part is that a non-dummy steam_api.dll is the solution as it restores compatibility that was already there natively.

I would be surprised if they did, it's not even marked on the game card as supported on XP. So at the point it's buyer beware, they only officially support it on GOG on 7, 8 and 10.

At some point GOG has to move on from outdated software, and so do users...

Best reply to this is by HyperinsomniacLive and found here:
https://www.gog.com/forum/general/release_fallout_new_vegas_ultimate_edition_1c381/post190

To some extent I agree... but GOG has every right to package extra files, etc. with games if they so choose. If they didn't have that right then the publisher would not allow it. GOG made it clear what OS's are supported and the game was tested with these extra files on those systems and passed testing (as far as we know).

If these extra files broke compatibility on "advertised" supported OS's then users have every right to complain when running those supported systems. But trying to play on unsupported systems is still at the end of the day buyer beware.

It's not on GOG to fix or even tell you how to fix the problems caused by these extra files because they never sold it to you on the promise they would work or be supported on XP.

Whether or not those files "should" come packaged with the game is an entirely different argument...

I think we need to take into account as what GOG started: keep all your installers for the future, they belong to you. Heck, they did that awful stunt to show how important it is to keep the installers, just in case GOG is gone.

Please don't focus on WinXP here, don't just think that maybe some old grumpy guys can no more use their old OS anymore. See it more like there is something changed of the game that compatibilitywise it's not working in the original environment anymore and so fixing it in the future by yourself on newer operating systems makes it harder, in a time when gog and especially Galaxy with the servers are also gone.

I think no one would complain If they need to break the compatibility of a game to make it playable for newer systems. They did that btw with Fallout 1, which isn't working with Win98 anymore. That's fine. The problem here is that this is not necessary, as there would be another workaround to have all the online features but also better preservable installers: just let Galaxy patch the games after installation to add all the online features. If you want to use online services, why not just get them online afterwards? Doing so you can keep the integrity of the installers as close to the original as possible, which is good for future compatibility, but also can attract folks who just enjoy the easiness of a client.

We all have a "right" to be dickheads, but that doesn't mean we should or have to be.

It's entirely possible GOG didn't even add them in this case. Bethesda could have. I very much doubt GOG added the achievements unless they somehow made their online API for achievements interact with the Steam equivalent without any changes to the game needed (not likely). If they did that would be pretty cool actually, but If not that means Bethesda did some actual work on the game to release here (which would be a surprise) by adding the Galaxy API using the Galaxy SDK.

GOG probably just grabbed whatever Beth gave them and packaged it up for release. Also it's not the like the original doesn't also come with unneeded crap in the vain of Steams crappy dll files as well. Either way, unneeded files are still present in the games build with or with out GOG's involvement.

Just my 2 cents, I have no bone in this fight.

Many things are possible and certain things are more likely. Normally GOG does all such stuff - fixing, porting, patching and packaging. I see no reason why Bethesda should play around with Galaxy-API just to add some features for an old game for a relative small group of new customers. GOG is the one who wants to push Galaxy and all the new features and they can do that best by adding the this new features to the most popular games.I can understand that and in general I have no problem with that, they want to increase their customer base and need to push galaxy. To me it seems that the offline installers become more and more to feeders for Galaxy and is not necessary.

Well, I think most Linux users would have moved onto a 64-bit OS.

They can bundle Windows XP version in "GAME GOODIES" section on game's card in Library. I see no problem here.
Stronghold HD, for example, offers "Classic Stronghold (Windows) and Classic Stronghold (Mac)" in it's "GAME GOODIES".

Why not include "Classic Fallout: New Vegas Ultimate Edition" as an extra?
It won't have achievements and Galaxy integration, but who cares? Galaxy doesn't work on XP anymore anyway.

It'd be great if GOG could offer the original (unaltered) files of EVERY game as bonus content. Without any fixes, custom installers or Galaxy stuff. For all the Linux users, people who have their own DOSBox installation, people who still have their old Win 95 (or 98 or whatever) machines, people who love to play around with the files on their nerd-systems, ect.

I'd probably never need them. I love that I don't have to do shit to get Wing Commander Privateer to run on Windows 10. But I know that there are a lot of weird people with even weirder computers on GOG. And I think it wouldn't really hurt GOG to offer those files too.

Be careful. There's a whole sect who think anything new is bad. They're like the Amish of the internet. They found their niche and they're not coming out of it for anything. They also get upset when their obsolete system isn't supported by something, or when something new is released.

I guess that preservation is just the focus of a certain type of pirate.
GOG doesn't seem to care about it anymore.

Remember the old abandonware sited that removed their entries as soon as they were available on GOG?
They had unaltered files and this was actually pretty neat, so was GOG in the beginning.