What a bummer Dunko!
When I first got my boards I was paranoid re vent plugs.I had a sign on cardboard -vent plug! that I kept in my board bag on the footstraps so I was reminded everytime I opened them. I got into the habit of screwing up the plug everytime I put the fin in but today I didn't..

I think I thought I'd wait till the last minute as the board is blue & it was out in the sun so i didn't want it overheating..
I went out for an hours sail and came in and realised the plug was in but not tight..@##$

. Luckily it was only 5-7kts of wind and flat water and I didn't plane so I don't think much got in..
Someone once told me to suck on the vent to see if there was any water in it so I did and there was a little bit first go but none after..maybe water sitting in the top? Anyway I've put a wick of tissue in it after taking the plug out and the wick is dry ??..Serious lack of hot sunny days here this season so I can't see me placing it out in the sun to evaporate much.
How do you know if there is enough water in it to be a problem? What sort of extra weight can it get with water and if it delaminates can it be fixed by a repairer?
I don't really feel its a big issue today as the wind was so light and the sail a really dry one but I'm interested.