Are these vegan, ie no suede/leather?

They are vegan.

We will be visiting Kauai and need a good all terrain shoe for beach swimming, tubing, zip lines and some trails. Will these work well for all?

These will work great for the activities you listed. They are lightweight and durable enough for you to fully experience the activities on the trip.

How much do these shoes weigh? Every ounce counts for this backpacker

These are the perfect wear for backpackers
Each pair weight around 450g ( size 7# )
Hope this will help you .

This may be a weird question, but do you wear socks with these? If so which ones?

If you head for some water activities, there is no need to wear socks. It allows the water to flow through them, and keeps your foot dry and comfy.

I have foot pain issues, needing a water shoe for a Water Amusement park, would these be comfortable for that?

These can provide the most arch support and cushion that you have ever found in a water shoe. It is like wearing a good supportive sneaker but for the water. The sole is pliable and comfy but very strong. Recommend it!