I know very little about MIDI files, and neither does my wife. She wants to shorten one so that she can use it as a ring-tone for her cell phone. Audacity doesn't do it. Will Sound Forge or Vegas (6)?
The Sony product that edits MIDI files is called Acid. Neither Vegas nor Sound Forge will do it.
You may also find some free utilities that support basic MIDI editing functions.
It is important to understand that a MIDI file is not an audio file -- there is no sound contained in it. It is merely a set of instructions (switches) that controls a device that produces sounds, called a synthesizer. Some cell phones support MIDI ringtones, some support Audio ringtones, and many support both.
Your easiest solution may be to play the MIDI file on your computer while recording in SF or Vegas. Then you can edit the recorded file and save it in an audio format compatible with your phone.