There are any number of inexpensive ground plane antennas designed for scanners, etc.
that will work just fine on 2m, even for transmitting at reasonable power levels, that will
be cheaper than buying an NMO base, whip, and radial kit.
(Those who wish can make one quite easily, of course.)
Most have a coax connector coming out the bottom. This actually makes for an easy
installation:
My vent pipes are standard ABS pipe, but you can do this with other types as well.
Get a "Y" fitting that looks like this that fits on the top of your vent:

Stick it on top of the vent pipe with the angled leg going down to one side. Stick the coax
up from the bottom of the angled leg and out the top, and attach it to the ground plane.
Then just set the ground plane on top of the fitting - if you want to get fancy you can bend
the radials so they run horizontally to the edge of the pipe then angle downwards.
And... that's it. The weight of the coax holds the antenna in place (you can secure the
coax to the vent pipe if you need more retention strength.
If you want it higher, just use an extension of ABS pipe and a coupler between the top of
the vent and the Y fitting.