I'd like to know if I should paint the plywood walls of my viv first and then do the coats of polyurathane, or do the poly first and then the paint? Also, what type of paint should I be using?

Use a non-oil base latex and paint first.

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Ken
"Some people spend an entire lifetime wondering if they made a difference in the world. But, the Marines don't have that problem." -- Ronald Reagan

I agree. You want the paint and everything else to be under the sealer

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Merlin,
What's Life Without A Little Magic!

I would first use a primer on the plywood. Without it you run the risk of peeling. Especially if there is humidity present. Then with 3 coats of a good quality outdoor paint you shouldn't need to put any poly on it.

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Mike
"Never worry about numbers. Help one person at a time, and always start with the person nearest you." - Mother Teresa