I'm looking into building a Glock 19 from the polymer80 kit.

Probably going to buy the Brownells RMR cut slide.

Since it is already cut for an RMR can a Vortex Venom be mounted there instead.

If I need an adapter what adapter do I need to get?

RMR uses a different mount than the Venom.  If they are MOS plated you can use the Venom as it comes with the plate.

This is one thing that kind of ticked me off about the whole pistol RDS is that the pistol makers and RDS makers did not come together and make one damn style mounting point.  Then the manufactures would not have to make different plates they could just cut the slide for the 1 mounting style.

You could buy a Glock 19 MOS slide. If you buy it as a full kit it will come with all the plates for mounting the variety of red dot sights available today.