A couple of things:
1) The posts themselves may be damaged or worn out. O-rings just fill a gap. If the gap is to big, even new ones aren't going to do any good.
2) The connectors may be worn out - same issue.
3) The posts may not be screwed onto the keg tightly or sealed completely.
Soap and water (as in bubbles) are your best diagnostic tool here. There should be no way you can tug / push / pull the connectors that makes them leak without getting nutty.
You mention vacuum grease. If that's the stuff we normally use on high vacuum fittings it's not rated for food use. The stuff you should be using is normally called keg lube. Keg lube is dirt cheap and easy to find (unlike the high vac grease). It's not your problem leak wise, but it's something you should be aware of.