"free" can mean that it costs nothing, is not restrained or is not occupied/taken. Take these examples: "That man is free" (He is not under restraint) "Is this free?" (does it cost nothing?) "Is the room free" (free = available) However, "vacant" ONLY means empty when used to describe an object (colloquially it can mean "stupid" or "unfocused"), and so you can only say: "Is the room vacant?" (Is it not occupied/empty?) In speech, however, we would normally say "available", "free" or "empty" hope this helped!

On a bus, "Is this seat free?" means "Is anyone sitting here?"

I am assuming that we are talking about apartments. A free apartment is when someone has let go of the lease and you can put a hold on it until the people move out or wait till it is cleaned and ready for lease. It does not mean you can live there for free. A vacant apartment is when no one lives there anymore and it is ready to lease and move in.