How do multiple death triggers work with a board wipe?
Both Vastwood Hydra and Hooded Hydra have leave the battlefield triggers. As their name implies, they will trigger only when the creatures actually leave the battlefield.

What that implies is that you cannot move the +1/+1 counters off Vastwood Hydra and into Hooded Hydra because (assuming, by your question, that they will both die at the same time), none of the hydras will be on the battlefield by the time Vastwood Hydra's trigger resolves.

You can, however, put the triggers into the stack in any order you want. That means that you may put Hooded Hydra's trigger on top of Vastwood Hydra's, get your snake tokens when it resolves, and then resolve the trigger that puts the counters onto those tokens, so the answer to the second part of your question is yes, as long as you stack the triggers properly.