Putting something over my vent stack would be a great solution.
What band do you need for the repeater?
What are your vent pipes made of?
Making a vent pipe antenna is fairly simple, especially if your vents
are black ABS pipe sticking out above the roof. A quarter wave ground
plane for 2m is only 18 - 20 inches tall, and you can get a 2 foot piece
of ABS pipe and use it to extend your vent, then put whatever antenna
you want inside.
If you run your coax up the vent pipe, make sure you have some sort
of a trap at the house end so you don't get sewer gasses in the house.
You can also make a dummy vent pipe, using the flashing fittings that
seal to the roof (and perhaps a few extra shingles to hide the base)
and stick it anywhere on the roof you want - rarely does anybody
actually loop up at the roof, and black vent pipes don't attract attention.
If you want to get creative, you can get aluminum stove pipe
sections and make a fat vertical, even with a top piece on it. Take
a stroll through the plumbing and roofing sections of your local
Home Improvement store and see what they carry. Our roof has
sheet metal covers that fit on the roof, with a hole for the pipe
to come out, and that would make a convenient ground plane
(depending on the band you want to use). If you have sewer
gasses venting up the pipe, then just tape a wire to the outside
of it rather than putting it inside.
There are designs for (mostly) horizontal antennas that are still
vertically polarized. For example, a loop about 14.5 inches high
and 29 inches long, mounted in a vertical plane, will give vertical
polarization and a 50 ohm impedance when fed in the middle of
one side. You can go even shorter (and wider) than that, but it
requires a bit more fussing to get a good match.
If your window faces towards the repeater, you can try just
hanging a ground plane on the inside of it with a suction cup
(often sold for stringing Christmas lights) and using just two
radials so it hangs flat against the glass, which can be behind
a curtain. (This might not work as well with some types of
passivated glass designed to reduce heating from the sun.)
You'll also be surprised how invisible a black quarter wave
whip is when mounted on a balcony, especially when disguised
as part of a flower pot stand, umbrella, or other permitted item.