Dynamic Ram Preview default is ~ 850mb depending on the Vegas version you are using.
What is happening when you render is that it buffers to that amount and having nothing left to use decides to kill itself off.
Tools > Preferences > Video (TAB)
Personally I use 4000 MB allocated because I have 8 GB of ram, I could just as easily used 6 GB or 7 GB but a rule of thumb in rendering generally is to use half of your blocks (especially since it's only your preview box). Using 0 is the alternative if your computer is really laggy.




