I bought fallout 3 on steam without knowing it was incompatible And it didn't work (crash in 5 mins or less). I tried everything to make get it to run properly (ex: changing the # of processors in the .ini, adding that .dll to the fallout3 folder)

This game though is compatible with Win7 but I am still hesitant on buying because of the issues with fallout 3.

Has anyone with a win7 x64 gotten new vegas to run that couldn't get fallout 3 to run?

Yes. There's a thread on the first page discussing this very thing. You might want to post a dxdiag so we can see if it's anything else.