cable routing for solar - vent pipe or drill roof?

Size the hole in the vent pipe large enough to accept either a weather-proof box entry fitting or a grommet. I'd use the latter since it is a friction fit. Lead your wiring down through the vent pipe and through the grommet,leaving the grommet around the wires, loose, outside the pipe. After you have the correct length pulled, slide the grommet over the wires and into the hole on the pipe. Apply liquid sealant to the grommet to insure sealing.