I think until you get the chance to really dry out your board, I would strongly suggest that you use the vent plug all the time.
Loosen it whenever you aren't going on the water, and tighten it whenever you do go on the water.
This "cycling" of the vent plug can do alot toward getting the moisture out and ensuring that your board does not suffer any internal damage from heat build up.
If you know anyone with a vacuum pump and regulator, that's the best way to dry one out, but you only need a partial vacuum.
I don't know the numbers but as you can see from Eva's website, she discovered the hard way what too much vacuum can do. She had a customers board "implode".
So a mild vacuum with a regulator and a relief valve should be perfectly safe and Eva might give you the safe numbers if you ask her in an email.
Hope this helps,