I think 50 would be way to high for the weight of a VX, ride would be pretty harsh. Personally I'd start out around 32 psi and see how that works for the street. Personally I've never done the chalk thing either, I guess I'm to lazy. I just keep a close eye on the tread and if it starts to wear uneven I adjust the pressure accordingly. Don't forget that the sticker on your door jamb is going to give you a recommendation too, that is only for stock size and ply tires though, and typically set for ride comfort over efficiency, so take it with a grain of salt.
As for off road, the bigger a contact patch, the better traction you have. But go to low and your likely to pop the bead off the rim. Honestly most of the time I don't even bother to air down when I go off-road and I do just fine unless I get onto rock. When I do occasionally do air down I never go below 15 psi.