What is the finder's default midi player and how does it know which instrument to associate with tracks?
If you mean Quick Look, it uses Audio.qlgenerator:

$ qlmanage -m plugins|grep midi
  public.midi-audio -&gt; /System/Library/QuickLook/Audio.qlgenerator (555.5 - loaded)


Information about what instruments tracks use is part of .mid files. .mid files predate the VST format, but there are 128 instruments defined by General MIDI and the sounds for them are usually synthesized from wavetables in soundfont (sf2) or DLS files. In earlier versions of OS X you could change the soundfont from System Preferences, but it's much more difficult now.

If you want to use .mid style sounds in a DAW, there's the DLSMusicDevice AU:



In Live you can change the sound by changing the program and playing at least one note from a clip: