kevthebassman, what vacuum?
Any vacuum. Space is probably the best one to use for purpose of demonstration. In outer space, a projectile can be driven to the speed of light if it's got an infinite amount of energy behind it. The faster it goes, the more energy it carries. However, when you factor in drag caused by passing through the atmosphere you will come to a point where the projectile cannot stand the force of that friction, and it will break up or vaporize (or both) from deceleration and heat caused by friction.
That's why meteors whizzing through space vaporize and break up in a bright streak of light as soon as they come in contact with our atmosphere.
That sort of thing doesn't happen to bullets though. Meteo speeds are measured in miles per second, bullets in feet per second. The only thing that really limits a bullet (aside from the case and gun it's being fired from) is it's construction and rifling. Spin a bullet too fast and it'll turn into powder. The Army uses a smoothbore gun on their Abrams tank so that they can push a heavy depleted uranium dart to 5700 fps.
Bullet mass gets you penetration not speed. In the Linebaugh test he showed a 335gr LFNGC bullet at 1050fps would get 38" of penetration but pushed to 1500fps it only gained 2" of penetration! Newtons laws of physics the faster it starts the faster it stops. Good luck.
Addressing this: I think I know what you mean, but bullet mass PLUS speed gets you penetration. If I take a 240 grain .530 lead ball and launch it from a slingshot at 250 fps, it'll leave a bruise. A 1/2 pound ball at a similar velocity will break bones and cause death, but it will not penetrate. When I take that same 240 grain, .530 ball and launch it at 1600 fps, a funny thing happens... that ball will penetrate all the way through the boiler room of a 200+ pound deer.
Now, I don't know what happened in that linebaugh test, but I'm guessing it had something to do with bullet construction. Maybe the 1500 fps bullet deformed on impact, dumping energy more rapidly and imparting more of it on the tissue (or ballistic medium, in this case) it passed through, resulting in only a very small increase in penetration. I think that's what you were driving at... correct me if I'm wrong.