Never ridden a Vmax, but off-hand I would say expect much less weight, much better handling. You're going to sacrifice quite a bit of comfort, in addition to some power and torque. The 919 should be quicker, in spite of lower hp, courtesy of a dry weight about 200 lbs. less than the big Vmax.
The Vmax is much more cruiser oriented, even if it is stupid-fast. If you're wanting something in the cruising vein, you might want to hang on to your Yammie. If you want a do-anything, quasi-sportbike, retro bike, go for the 919.
2002 919 40,000 miles
"racing is life...everything else is just waiting"