Cover the vent
They sell magnetic vent covers at nearly all hardware stores. Buy one, and cover the vent when it's not being used. This will prevent moist air from entering the vent, though it may not stop water if it pools around the vent.
Install an exhaust fan
Install a properly sized exhaust fan in the bathroom. Let it run during showers, and for about 20-30 minutes after showers.
WARNING: Make sure you vent the exhaust properly.
Sizing an exhaust fan
Exhaust fans are sized using Cubic Feet Per Minute (CFM), so you'll have to first figure out how many cubic feet your bathroom is. To do this, measure the length, width, and height of the room.
- Length = 10 ft.
- Width = 8 ft.
- Height = 8 ft.
To find the cubic feet, simply multiply these numbers together.
10 ft. * 8 ft. * 8 ft. = 640 ft.3
Next we'll divide the cubic feet by 60, to figure out how many CFM is required to exchange the air one time in an hour.
640 ft.3 / 60 Minutes = 10.66 ft.3/Minute.
It's recommended that the air in a bathroom be exchanged 8 times per hour, so you'll have to multiply our CFM value by 8.
10.66 ft.3/Minute * 8 = 85.33 ft.3/Minute
So in this example, we'll need an exhaust fan with at least an 86 CFM rating.
Quick formula: (Length * Width * Height) / 7.5 = Required CFM rating.
Making sure the fan runs long enough
When installing an exhaust fan in a bathroom, it's a good idea to put the fan on a timer. This allows you to run the fan for the recommended 20-30 minutes after a shower, without having to worry about remembering to turn it off.
Warning: Make sure you get a timer that is rated for motor loads.
Glazing manufacturers have spent decades perfecting a spacer system that does not eventually leak and condense. They still haven't completely perfected it, though they are much better than 20 years ago. There is no way you're going to put together a system that does not eventually leak, especially with a vacuum. You'll have a better chance with inert gas, but I'm not convinced even professional argon filled glazing stays argon filled years later. It's not that I doubt your fabrication skills, it's just that it's a very difficult problem to address.
For all the time and effort spent assembling a window system, it's worth buying professionally built glazing units. If you want to save some money, forgo the argon filled and simply get dual glazed units with a low-E coating. IMO, these offer decent thermal performance and good value.
Best Answer
I highly suggest what I would call the 'board up method' I've seen in places, but executed classier of course. I would create two rounds (squares would also work) roughly an inch larger than the diameter of your hole. I would suggest some nice 1/4" plywood or similar. Then drill a hold in the center of both pieces and use a bolt and nylon locking nut of sufficient diameter to pass through the bits you've cut + the thickness of the glass.
Then with a little help from a friend or a particularly sticky piece of tape, have one round on the outside of the glass, one on the inside and both through the holes of the cut pieces and gently tighten the bolt.
Then you end up with not a plug so much as what I'd call a "cap". Pieces of felt, cloth or similar would help keep the pieces sealed to the window and prevent any scratching.
These rounds could then be painted to be less obvious and help prevent them from weathering in the even that they were rained on from the outside.