One way to reduce the number of holes is to attach ONE beam with 2-4 bolts. Then screw every hook into that beam.
I think you said "second story deck. And hanging vines off the deck?
Would you pick them from below (ground level), or by hanging over the deck?
You might use plastic to keep water off the deck. But it still has to run off somewhere, and dripping onto the deck below yours might still be a bad thing.
Good luck! I also have limited sunny space.
