Not quite sure I understand your problem but I replaced all four of my vent caps with the new style that swivel with the wind. Can't recall what they call them BUT,
Each one comes with a bezel that slips over the 1 1/2" pipe to take up the void between the 1 1/2" vent stack and the hole in the roof which is about 2 or 2 1/2".
Slide the bexel in place over the black 1 1/2" pipe.
Now slide the new vent onto the top of the black 1 1/2" pipe. Then, lift the bezel up so you can put a couple of small screws (supplied with the new vent) through the new vent and into the black pipe to secure it in place.
Now, liberally apply Dicor Self leveling Sealant around the Black 1 1/2" stack and then lower the bezel in place over the Dicor.
Press the bezel down into the Dicor sealant.
Now, apply Dicor sealant around the OD of the Bezel and around the ID of the bezel next to the OD of the new Vent.
You are done, and sealed against huricane force wind or rain.
Job well done.
Dennis & Carol
2003, 4006, 41' DutchStar,
Spartan, ISC 8.3L W/BanksPower - 2013 Honda CRV, BlueOx Baseplate, Aventa Bar & Patriot Brake. And the 04 Bird, Sunshine Car.