My prediction is that the E-Jet is going as part of a strategy to have VARA do all 'regional' flying, as their name suggests. That includes 100 seater aircraft. Using the 737 to fly 100 seater routes is just a stop gap while they set up Alliance F100's in the East Coast to do what the Skywest F100's are already doing on the West Coast; replace the E-Jet flying.
So by 2020+ maybe Virgin will look like this…
Wide-body: B787 (A330 and B777 gone)
Narrow-body: B737 NG & MAX
VARA: ATR and F100
Tiger: B737NG
The only caveat to this prediction is if they follow the QF playbook, the 787 and MAX's could go to Tiger first….after all, the LCC needs the most fuel efficient aircraft.