Not survivable....
Yes... I know, I know. It's a movie - don't tell me that. And it's a great movie.
But the ending job where Frank cuts a hole in the vault with a burning 8,000 degree metal rod, for several minutes, would make for untenable conditions.
Not only could you not stand that close to an 8,000 degree flame, it would heat up the room to hundreds of degrees in minutes.
Also, something that burns that intensely, eats up a ton of oxygen. As do the CO2 extinguishers displace oxygen.
The hear combined with the lack of oxygen and noxious gas given off certainly would have killed them.
Nevertheless - still a great movie.