cubase or logic both great sequencers that work with audio & midi (altho cubase aimed primarily at pc and logic is mac only).

VSTi or soft synths are loaded into cubase and yea you can play them via a controller keyboard....but you dont have to. You could manually program simple MIDI part that the soft synth will play. Take a look at the MIDI effects topic for twisting up MIDI parts on cubase. Once you got the part as a MIDI you can mixdown to an audio file.

Once you decide on your sequencer you'll discover your preffered way of making music. Good luck with it ;)