Previously I begans posting under other thread "was stable but now crashing". Thread now locked so I am just beginning this one.
In short, after 100 hours with minimal glitches and crashes, I eventually encountered a relentless crash to desktop after exploring Vault 3.
First noticed in Vegas ruins, I encountered this crash EVERYTIME I entered into combat. I was able to avoid CTD by quickly turning back into Fiends Territory. I was even able to fast travel to other locations, barter, see MD, sleep etc. But as soon as my companion Boone and myself engaged in any enemy combat, the result was an immediate CTD.

After retarting FONV at least 10 times I wandered to this forum to find the substantial amount of similar issues other PC gamers were having. And actually felt fortunate that after so much gameplay that this was the first severe issue I encountered, considering the degree of issues others were having. Needless-to-say, I began reading and troubleshooting steps as suggested in others' posts and threads. But still I continued to experience the dreadful CTD.

Today, HOWEVER!!! I seem to have had a "slight/relative" breakthrough. As I mentioned before, I could continue gameplay until having to battle. By a long shot, I told Boone to "stay here" and journeyed forward on my own to see if I found similiar results. But much to my suprise, NO CTD! I have been able to continue on my own for almost an hour and half, into previously unexplored areas killing upwards of 30-40 enemies (both iron sights and VATS) and still no crash!!!
Unfortunately, leaving my companion behind definately reduces the allure of FO but perhaps this is a step in the right direction in determining a cause of the bug and/or the crash to desktop.
If anyone else experiencing a simliar CTD following their Vault 3 visit (or any part of the wasteland for that matter) please attempt this and see you have any success similar to mine and post any results you may have.
Optimistically, this might be of signifcance...

**my dxdiag is posted in previously mention thread

Windows 7 64-bit
Intel Core2Duo E8500
GeForce GTX 280 (driver 260.99)
8 GB DDR2

See existing thread on save corruption. Includes numerous variations of play which seem to simultaneously cause repeated crashes for some while not for others. I, for one, have managed to get save corruption with and without companions in tow at both Vault 3 and REPCON.

i had the same problem. leaving boone behind did fix the ctd's. i've since sort of abandoned that game line and plan to start over with a properly patched version, but someone else on here suggested using "tai" from the console. that may be somewhat less interfering in some situations for you, and i think it would do the same thing or possibly be more effective.

to get the console hit the "~" key, just type 'tia' and then "~" to get back out.

I wasnt sure if this was actually and instance of save corruption, since I am able to fully load the save, and didnt encounter the crash until any attacks began.
I am certainly a novice at best when it comes to the tech aspects of the game and programming (something Im trying to improve) thus I wasnt sure how this situation related to others.
I am going to attempt using "tai" from the console and see what happens.

it seems the bug has something to do with Boone/companion firing a weapon. I instructed him to use melee attack, and oddly enough we continued in combat without the previously normed CTD.
I had returned to him after venturing out for a good hour, curious if for some reason we'd be able too continue on w/o the ctd. Predictably enough, once again when we engaged in battle instant crash. Shortly after I tried the melee approach and that seemed to circumvent the CTD. Its not a fix, but at least a workaround.

So, he's choking on the ranged weapon...?
Have you given him an unusual gun + ammo?
Try without giving him anything at all...
(I retired my Boone ages ago. He's still stalking around Novac in the Metal Armor I had given him.
I didn't kill a thing for days, when he was with me...)
Regards

Its kinda ironic you ask about ammo. I noticed the CTDs firsy happened after I first started using the workbenches to Overcharge the MF Cells for use with Q-32 matter modulator ad/or plasma rifle. I initially thought that was the cause for the crash seeing that the ammo had a +1.50 gun cond when the gun already at 100%

I had him carrying 5 different guns, with more than adequate ammo for all of em (yes, including modded ammo ie. 12guage slugs, armor piercing 9mm etc.) But for some reason when toggling inventory, despite carrying the corresponding weapon ammo, when highlighted, all but one gun was listed with its respective ammo at 0/0.
For example, I wasnt carrying any .308 weapons, but had Boone holding the Hunting Rifle with 130+ (.308) rounds. The same applied for 9mm submachine gun, and any plasma or laser weapons. The caravan shotgun was the only gun that showed he was holding ammo for.
I ended up returning to a shipping box to to stash the arsenal I had him carrying for me, in case I had ditch him again. The palatoaster (I think its called) was the only companion equipped weapon I had success with in order to avoid that wretched CTD. I also thought it was odd that at the same point I noticed this glitch, he stopped wearing his 1st recon beret.

On a side note, I dont know if its a built in feature/option for the game, but I tried equipping him with antivenom follwing a scorpion attack, but he wouldnt use it to avoid -HP effect of the poisoning. But as I mentioned, even if the game was relatively flawless, Im still not sure that he'd use it.