PUt a piece of saran wrap on the muzzle of your rifle before putting the patch and round ball on top of it, Cut them off together when you seat the ball, so that the plastic wrap acts as a sealer to keep water from running down the barrel and past the ball and patch.
Keep the barrel pointed down , with the lock under your armpit. I use a pancho for rain gear, and keep the poncho over the rifle, without the muzzle sticking out. Use some tape on the muzzle to keep water out of the barrel. d
I prefer using beeswax to create a dam in front of the frizzen to keep moisture and water from running down into the pan, or, I suppose, into the vent. I think a good idea for protecting the vent, and the powder charge from moisture getting in through the vent is to soak a cleaning patch in rubbing alcohol, and put half of it on the pan, held down by the frizzen, and the other half up against the vent and barrel. The evaporation of alcohol will wisk away any moisture that has gotten, or will get into the chamber through the vent hole, and the alcohol will displace any water in the patch itself. Take a small ( 2 oz.) bottle of alcohol with you so you can renew the alcohol in the patch during the day. This does mean that you have to prime your gun when you hear or see game approaching, but in a down pour, unless someone else is moving towards you chasing the deer out of their beds, the deer might not be up and moving about at all in the rai.
If you are going to stalk the deer in their beds, then do as RoundBall has indicated, and that is change your priming powder every 15 minutes, and use grease to seal the edges of the frizzen and pan to keep moisture out.
Be sure that whenever you wipe down the frizzen and pan, that you also wipe down the flint, both top and bottom. Moisture can condense on rock, too.