Interesting discussion, again immediate reaction is no, however in some circumstances maybe they should.
It should probably be based on length, ie, there is little point in a 8 foot surf SUP being classed as a vessel as it is much closer to a surfboard. A 14 foot or 17 foot Downwind race board which can travel along at more than 8 knots in the right conditions maybe should be a vessel, all gets a bit grey really.
On second thoughts, No, they shouldn't be, it's just too hard

DM