Hmmm.... I posted something a couple of hours ago, and it didn't post here...
Oh, well, I have a better idea for your to try now....!
Here's a website with a drawing of a rubber washer that will seal your board
tightly if you can find the correct size at your local hardware or autoparts store.
If you can find a rubber washer or "square ring" that's a close to a snug fit on the outside diameter and fits a 9.0 mm thread on the inside, you will get "double sealing".
As you snug up the threads of the vent plug, the rubber washer/square ring will compress from the pressure of the threads , forming a tight seal between the counterbored flat in the vent plug fiting in your board and the underside of the head of the vent plug.
The square ring/rubber washer will also "expand" radially to seal on the outide of the plug threads and the inside of the head clearanced counterbore in the vent fitting.
Also, I think you may be trying to tighten the vent plug a little too tight and are "crushing" the new O-rings.
You only need the plug to "seat" lightly to seal. Finger tight is not enough, but you just need to "snug it" enough so it won't fall out.
Look for a fiber gasket that will fit in the hole and that has a 9 mm inside diameter.
That would work even better.
Hope this helps,