You are protecting the contacts from condensation which is from moisture in the air. you displace that air with Vaseline in the socket. When you seal off the socket, there is still moist air in that socket (albeit small amounts-but its there regardless)
Always put that stuff in the socket. cleans up will with a hair dryer, just up-end the board and heat. most will melt off. and acetone on Q-Tipps will help with the rest.