First deck build, what do you all think of leaving the bottom open?

The critters thing is real, not just something people say. I left mine open for two years and ended up with a family of groundhogs under there, plus mice and every neighborhood cat using it as a hangout. Skunks are the nightmare scenario since they'll den under a deck and you can't exactly convince them to leave politely. Closing it in is cheap insurance compared to trapping and relocating something smelly in the spring.

Do both, sort of. Close it in but don't seal it airtight. I built wood lattice on mine instead of vinyl because I figured it'd look better against dark railings, and I left a few inches of gap at the top and buried the bottom panel a few inches so nothing digs under. On my current deck, a 14x20, I put hinged access panels on two sides so I can still get under there if a pipe or the ledger ever needs looking at. Skip the cheap thin vinyl panels, they sag and crack after a couple winters. Solid wood lattice painted black would match your balusters way better than anything vinyl.

On the vinyl question, I'd skip it next to black balusters. I put vinyl lattice on my old place and it faded to a chalky gray within a couple years and got brittle enough that a soccer ball cracked a panel. If you can find it, the black vinyl looks way better with dark railings, or just paint wood lattice black to match. Also plan a access door somewhere, you'll want to get under there for the footings or plumbing someday.

This is helpful because I'm in the same boat, just finished mine in the fall and haven't touched the bottom yet. The buried panel idea makes sense but does it actually need to go down that far to stop something digging? I was thinking of just laying some of that heavy fabric stuff on the dirt inside before I close it up so weeds and leaves don't pile up under there. Also curious how you guys handle the leaves, seems like a closed bottom would just trap everything that blows in.