ReFX Vanguard VSTi cuts out if playing over existing track

Description:

I'm trying to add some synth effects to a song using REFX Vanguard via M-Audio Keystation keyboard. Its been working great - very cool sounds, however when I try to 'audition' (not record) vanguard sounds while playing the main music track in my project sometimes Vanguard will stop playing. I have to close REAPER than reopen it to get it working again.

Setup Configuration:

Latest Reaper software installed
Lexicon Alpha ASIO sound card
ReFX Vanguard VSTi synthesizer
M-Audio Keystation E49 keyboard
Routing to 'Master' is enabled in 'Reaper software'

Steps to Duplicate:
1) Opened a new project in Reaper
2) Imported an audio wave file
3) Inserted a VSTi track
4) Hit the 'FX' button and add the 'ReFX' Vanguard plugin under 'VSTi'
5) Hit the piano keys on the M-audio keyboard - sound plays successfully
6) In the Reaper main window hit the 'Play' button
7) While song is playing hit keyboard keys

Expected Result:
ReFX Vanguard should play over an existing track to 'audition' samples
before recording.

Actual Result:
the MIDI track containing REFX Vanguard cuts out. The track meter still
detects the input but its colored entirely in RED. The track I imported originally still plays.