I think the VST will need to be able to import/drag a tempo mapped midi from the host into the plugin track for tempo changes or if it was like a live recording that was mapped after.
(ACW is good for creating Drum Tempo Maps, just open a drum track from a live recording with the ACW in RealBand, then you just generate a Metronome track up, then save the Metronome Midi track, open the midi in a new session in your DAW then drag all the live tracks in.)
This will let the plugin get an accurate map for RealTrack/Drum/Midi generation rather than following the tempo changes up as the host plays.