Long story short though I can provide the long version if needed:

Got a new mouse to play New Vegas with, wanted to play but wanted to get the vanilla version going while still retaining my mod info somewhere so I could finish some modded playthroughs later, decided to copy paste the game files onto the desktop before uninstalling and reinstalling, since then the game won't run. I've reinstalled thrice, both with steam and a hard disk, I've done everything the steam support page for the issue suggests (https://support.steampowered.com/kb_article.php?ref=8676-EFLX-7453&l=english) and through it all I've purged my computer completely of New Vegas files, including my modded copy on the desktop. Tried running the Nexus mod manager just to see how it reacted. Rescanned so it could find the new file destination (not that it changed, but just in case), and then the NMM refused to open claiming that Falloutnv.esm was completely missing.

What the hell did I do to break everything and how the hell do I fix it? Help pls.

My guess? You have a registry entry that points to an installed location that's no longer valid. Try doing a registry search for fallout...or maybe vegas would be better...and if it finds a key saying installed location, delete it.

Nope, it's even better:

Got my answer not from Steam support, but from googling. Know what it is?

Long ago, by a fluke, I'd accidently tricked Steam into unlocking the english version of the game for me. I own both the english and german copies, but bought the German first and that PERMANENTLY locks your account into German-only. Steam is then set to go kill the Falloutnv.esm file if you have it downloaded in english or any other language. I have no idea how I tricked it before, but it's back to killing the english version now.

Back to the drawing board... I've tricked it once, I can do it again.

EDIT:

HA! Take THAT, German government and Steam programming! Got the game running again and in English. Now to return to my home in the Mojave!

Hah! Well, haven't a clue. Good luck.