dirt or water in carb (clean out bowl or put thru some cleaner)
faulty electrics somewhere, HT lead/leads, wiring, battery dying, loose earth?
clogged air cleaner
most probs are either fuel,air,or electric related so try and figure which one of those 3 it might be
my 1st guess would be something to do with the carb
if you do a google search (vn800 stalling & other keywords) you might find something, or see if there's a forum for those bikes, & search the forum