Hello,
I can't get my game to start after this patch.

I've tried this
After updating to v 1.2.0, if you're having issues opening New Vegas, try re-verify the files.

1) Open Steam
2) Right click on Fallout: New Vegas
3) Hit Properties
4) Click the "Local Files" tab
5) Click on "Verify Integrity of Game Cache"

And Steam says it's missing a file and will find it but it doesn't work. I thought there was a thread about this and I couldn't find it via the search.

Please help

Just so you know, there is a thread detailing some techniques to fix this very issue, already. Use the search. It's called "Fallout Won't Start After New Patch". I had the same issue the other night when I went to boot up my game and after a little bit of fiddling, I was able to get the game working fine again (with all of my mods). Anyways, hope it helps you out, too!

They broke ArchiveInvalidationInvalidated!.bsa that's the most likely the reason your game crashes,
remove it from Fallout_default.ini for now.