It has been eating away at me for months and now finally after a week of trial and error (and about 100 crashes)
I changed 10 lines a day and then it suddenly worked... so then it was just trial and error after then...
I finally found the fix and can confirm it works!
Vault3a all fully rendered after using my fix
http://img156.imageshack.us/img156/4782/renderworking.jpg
The fix is actually simple!
Load up GeckCustom.ini located in your documents/my games/FalloutNV
and find the line "bUseMultibounds=1"
Change that "1" to a "0"
so it should say
"bUseMultibounds=0"
Save and fire up the geck...
And it works.