The engine has to be sealed up tight for a vacuum pump to really work. It takes some dedicated machine work to get an engine sealed up that tight. You also need to use the correct gaskets everywhere. If you have an air leak into the engine then the vacuum pump will just make a mess.

If you're thinking of getting a vacuum pump the first thing to try is to pressurize your crankcase and see how long it takes to leak down. Just put about 5 or 10 psi of air into the engine and listen for the leaks. If the engine can hold the air pressure for a little while then it is sealed well enough to run a vacuum pump. If the air blows right out then a vacuum pump probably isn't going to work for you.