Vegas does not record or play back MIDI tracks. It is audio (and video) only.
You can either use Acid (or something else) ,or simply render the MIDI track to a WAV (or whatever audio format) and import that to Vegas. If that MIDI track was 'final' then Bob's your uncle and carry on from there.
If that MIDI track needs subsequent editing (in whatever created it) then just make sure to render it to the same length so it slots back in exactly.
If you don't have MIDI software to render it, you may even be able to play the MIDI track back in a MIDI playing app, and similtaneously record it live into Vegas.