The vinyl siding is not considered water tight. It is a rain screen. It keeps the bulk of the water off of the house but siding must be installed over a water barrier like Tyvek or felt paper. All of your fixtures, windows and doors are flashed at this point. I would seal the pipe where it comes through the sheathing with a flexible flashing tape like is used around window openings or maybe a caulk type of sealant. If a fixture block could be found big enough that would be a nice finishing touch to run the pipe through, actually a meter block would probably be the right size. It is just a solid plastic block with an integral J channel that would give it a nice finished look. In short,think like a water drop when flashing, the water barrier is the membrane behind the siding, finish your siding in a way that diverts water from getting behind the siding but don't think water will not get behind the siding because it does. Vinyl siding has weep holes in it to let water out.