Grapes can be pruned any time (though the ideal time is early in winter
dormancy)... and a little dripping of sap at the site of cuts is of
little consequence. I'm guessing that your vines are already leafing
out. It is ok to prune them now but yes, you will see a fair amount of
dripping when you're done.
Dont' worry, grape vines, from the very first year, need to be
pruned...and pruned almost brutally. Aside from those long canes that
are allowed to remain for training and shaping purposes (as in working
them across an arbor or trellis, or along a fence), all other canes are
ideally reduced to 2" or 3" long stubs (with 2 or 3 buds on each)
.....every year after dormancy begins in the fall. If you were to watch
professional vineyard pruners work, you'd be genuinely appalled at the
extent of pruning!