I haven't ever used Reaper but there will be a way of pointing a new midi track in Reaper at your installed VSTs. In the event you can't get it sorted here (though I've never known LBH to ever give up), you might be better joining a Reaper forum and asking them how to get Reaper to use external VSTs.
As for 'setting the scene' being a guitarist, I'm a keyboard player. Keyboards are easy: you've got 12 notes a semitone apart which repeat themselves at octave intervals, I could teach a five year old to play 'Happy Birthday To You' in ten minutes using numbered stickers on the keys, because each key plays the same unique note every time you press it. On the other hand there's no way I'll ever understand how you guitarists, and other strings players, know where to exactly put your fingers to get recognisable music out of a few strings stretched across a piece of wood. What you do is voodoo. EDIT woodwind and brass players are voodoo as well, they have nowhere near enough buttons on their instruments to get all the notes they play. You're all magicians.