A system crash during a test like Prime implies overheating problems, a CPU hardware failure, or a PSU failure or overload. It tends to exclude problems with Vegas, device drivers, disk drives, or the dreaded Microsoft .NET.
Open the case and verify that all fans are running (including CPU, PSU, and GPU or disks if applicable). Turn off the computer and disconnect the power cord and blow out any dust in the case with a can of compressed air. Add up all the power requirements of all the components in your computer and make sure they don’t exceed 80%-90% of the rated power output of your power supply.
If it’s not overheating or an overloaded power supply, and given that you’ve tested all the RAM and it works, the CPU would be the next suspect. CPUs usually don’t fail unless they overheat, but it’s remotely possible.
In all of these cases, I’d expect other CPU-intensive tasks to crash the system, too. But there aren’t many things that are more CPU-intensive than video editing or video games, so it may be that you have nothing else on the system that puts the same load on it.
A blue screen or spontaneous reboot could be a software issue. If the machine simply freezes solid, it could be software or hardware.