I just got Sibelius 7 and I'm having trouble assigning vst's to instruments in the score.

I've had no problem adding Edirol and Truepianos as playback sources but I don't know how to assign each to a particular part.

I have no experience with either Finale or Sibelius so any help would be great.

Have you tried looking in the mixer window? I'm pretty sure you could change vsts there in Sibelius 5, but I have no idea about 7.