I use those nylon colored caps you can get at the hardware store.
They come in like 3/16", 1/8", 1/4", etc. sizes.
They just look pretty on the engine.
Seriously, I shoot some RTV into the fitting before slipping the cap on.
This is what keeps the fitting from leaking.
The cap is there just to remind me nothing is supposed to go on it.
If I ever want to reuse the fitting again, using a proper sized drill bit will clear out the RTV, and viola...
I don't like the rubber vacuum caps cause they look butt ugly. :P
-Ted