Resealing a vent from the top vs R&R

I think Dicor sticks better to a clean surface which could include other Dicor. I’ve scraped away Dicor to get to clean Dicor. I also wouldn’t remove a vent either unless it’s broken, and I had to.

I’ve used Henry’s on a house, but I wouldn’t use it on an RV. Not saying it could work, but it’s really messy compared to Dicor or Eternabond.

Per my experience - butyl tape is the only thing that seals permanently, or let's say 20 years.
All caulking on the top is a band-aid who require annual rechecking and even then some caulk would look good to the eye, when in fact it did not stick on 1 side and was channeling water inside.
Also I found Henry's roof sealer (about $50/gal) the only thing that sticks to everything and last if you want to recoat whole roof.
The roof sealer alone is not good joint sealer as it shrinks when dries.

That depends on whether you want to do it once, right, and have it sealed for 10 - 15 years or do it every year. Dicor poured on top of the flange is never done on a boat, with good reason - boats sink when filled with water, campers just slowly rot.

I would never remove a vent unless the roof is getting replaced.

I clean real well around each vent and then I blue tape a nice straight line square configuration around each of the vents. After taping I then use self leveling Dicor and work with a throw away brush working it back and forth well into the surface. This helps keeps it from building up so high and works it into the surface to help assure good adhesion. After working the Dicor to a relatively thinner area nearer the masking tape I slowly pull each piece of tape off, carful not to mess up having a nice clean line.

Should be good to go. I have been doing this to each of my vents twice since I bought new 18 years ago.