In Florida you have to spray every month and I do that. We've never had a problem with roaches before. I have sprayed the kitchen repeatedly and I think I have licked them. But I after being up on the roof working on flashing around the kitchen vent pipe I began wondering. This pipe is near a nearly flat porch roof. When I was up there I found the porch roof littered with all kinds of leaves and little branches and I also found there were ants wandering around up there.

Does the vent pipe have water in it that would block bugs from getting into my kitchen? Would anything happen if I rigged some screening to cover the opening of this roof vent?

Thanks.

Plumbing vents should NOT have water in them as their purpose is to prevent vacuuming the water seal out of sewer gas traps when flushing, draining etc. That liquid seal will prevent most insects from house entry. I say most since some larvae can hatch after being laid in such conditions.
If you are going to screen them, the screening cannot reduce the vent area.
The seal between vent and roof/wall penetration (usually a boot or flange) should be maintained. The best construction is strategic flashing. If caulk is used, remember it has a short service life.
Trying to keep the organic matter off the roof will help minimise insects looking for a meal or building material.