It's not something I'd recommend. The VRex is a very short wheelbase bike, designed around a relatively upright seat position, and the M5 seat is designed to be very reclined. I tried laying the RANS seat on my 'Rex back further and almost ended up on my head a few times. The weight distribution was so heavily rear-biased that just trying to start uphill was enough to provoke a wheelie. The fact that my seat is almost all the way back to start with probably contributed to the problem, as did my height (6'3"). Needless to say;, I'm back with the original seat position.

SP
Bend, OR