Code word used by homosexuals for getting your ass licked. Vegemite sandwich was originally popularized by “men at work” in their 80’s song “Downunder”.
There are numerous references to the homosexual community in the song. Read the excerpt below and see the “Real Meaning”.
"Buying bread from a man in Brussels
He was six foot four and full of muscles
I said, "Do you speak-a my language?"
He just smiled and gave me a vegemite sandwich"