Originally Posted by SquawkIdent

Not dumb at all! Just two ways of accomplishing the same thing. Never having done a base trade I'm not an expert but here's my understanding:

For a base trade, there has to be someone on the other side of the trade, i.e, one pilot swaps DEN to SFO and another swaps SFO to DEN. If you don't have a trading partner, the trade doesn't happen.

For a vacancy bid, nobody has to take the other side - you just lateral over the base you're bidding into on the vacancy bid and create a hole in the base you're leaving.