To be honest, I don't see how it could. The oil caps on these cars are not exactly air tight to begin with...
If I am wrong somebody please correct me.

 

The computer will make adjustments to compensate. If you can find a post by Injuneer about breathers, he explains it in detail.

 

Question. I have seen talk of a vacuum pump that attaches to this portion of the engine to increase piston ring sealing, and possible HP gains of 10-15. If the increase in vacuum would increase power in this manner, how do the breathers work?

 

breathers just hopefully vent the crankcase pressure. That puts them somewhere in between positive crankcase pressure (bad) and negative crankcase pressure (good).

 

Are the breathers more effective than the connection with the tb?

 

The connection with the throttle body is to let MAF metered air into the crankcase for the PCV system to suck it back out again and the engine burns it, so it is just an air supply. When you are making more pressure in the crankcase than the PCV system can handle, like on a blown engine, then you might need more breathing than the stock PCV setup can provide.