Ain't that how it usualy goes?
So why not just use a bit of this
CPES Warm Clear Penetrating Epoxy Sealer MultiWoodPrime
Which is smiths cpes, it will provide the waterproofness you seek. Then just either glue on the vinyl with some 3m adhesive, or manybe better yet glue it on to some very thin fiberglass board, then screw that into the bulkhead. Makes for easier replacement in the future. But of course the price then goes up.
If you can't make up you mind right now, just paint it white after faring it, perhaps that will do for now.
just noticed the date of the thread ... sorry.