Session Drummer 3

It sounds like previously you were using a Drum Map, and now you've changed the Midi Out for the track to direct to Session Drummer. (This will also change if the Drum Pane is being used or the Piano Keys in PRV, if you were editing here)
 
Go the Midi Track and select the Output Box (marked with an O) to DM1-Session Drummer (I think, I don't have it installed to see exactly what it displays to)
 
If you are using Simple Instrument Tracks, Open the Track Inspector, click the Midi tab at the bottom and the Midi Out will show (There are two outs in Simple Instrument Tracks, the MIDI and the Audio, so normally its the Audio that shows in Simple Instrument tracks) 

So do you get an option with these VSTs to open in Drum Pane or Piano Roll for PRV?
 
Let me expand my thinking, incase I have got something wrong. With a normal VST, if you have drum maps selected, in the PRV view, you can only see it in the Drum Pane, not in the Keys. If you do not have VST selected, then you can only choose to input notes on the piano roll in PRV