Quentin Tarantino has said that
Ving Rhames wore the
band-aid because he's got a scar on the back of his neck and they thought it would stand out. Why they chose to cover it with something that makes it stand out more is beyond me...
He's also said, "It's whatever you want it to be," indicating that it's not what's in the case that's important, but rather the fact that everyone wants it.
On a side note, I think Vincent was killed because he didn't receive the same "sign" that Jules did. Jules wanted to repent, while Vincent did not.