It's not a big deal because it's the top glass of your tank and not in the water, but soap is a big no no... too much residue left behind. Whatever you do, never clean the inside of an aquarium with soap, like if you ever start over. Like, if I were ever clearing out a tank to start over and someone (with good intentions, of course) cleaned of the glass with soap and I found out, it'd buy a new tank.
I agree with the other two, a mild acidic solution(the vinegar) will eat away at the base pH'ed crustiness on what you're trying to clean. Soap wouldn't work as good either way because soap also has a basic pH.
