The words empty and vacant mean the same thing in those examples, so you would only need to use one of them. Besides that, yes those sentences do work. When seats in the bus are not vacant, I stand and travel is grammatically correct.
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User_garyWhen seats in the bus are not vacant/empty, I stand and travel.There is no need for 'not vacant' or 'not empty'. That's more negation than necessary.
User_garySo we can't say empty or vacant for a machine?No, now that you mention it. Machines are unoccupied, I'd say.