Why Butyl vs Caulk under roof vent flange?

You can get the item back off! Dicor or others like it make removal very hard.
Are you using or was black butyl or grey butyl tape used?

I've only ever dealt with the grey

I think one reason is because it doesn't dry out and crack the way that many caulking materials do over time, but remains flexible and stretchable. It's also what the manufacturers of the roof vents, windows, etc. generally recommend for use, at least in the handful of installation instructions I've seen.

I dont think you would want to put butyl on rubber roof. In case the butyl tape has petro chemicals in the fromula you could be asking for trouble. On the roof I have always used a generous bead of Dicor, screw whatever down and more Dicor over the screw heads and around the edges.

I don’t put either under vent flanges. Dicore self leveling applied properly will seal flange.

Either will work for a roof vent but I would use the butyl tape. We did some testing of various ways of sealing airframes were I worked and all of our Blackhawk helicopters were sealed with butyl caulking between riveted skins. We also used this caulking to seal composite air frames. It is more flexible does not dry out or shrink like other materials. There is a lot of different loads put on these airframes in flight and things move, not on like a MH traveling on bumpy roads. I have seen 20 year old airframes come in for overhaul and repair and the butyl seams are still good in spite of rivets and Al skins being corroded. MH's do twist, and we have all heard of wind shields cracking when improperly leveled on some MH's, so that is why I think it is a good thing to use a flexible sealant like the Butyl tape.

enblethen is correct - caulk or lap sealant will end up gluing it down, making it difficult if not impossible to remove without damaging things in the future. It's hard enough removing lap sealant when it's just on the surface. DrewE is also correct - good quality butyl tape won't dry out in a long, long time. But beware, there's also putty tape which is sometimes sold as butyl tape, but isn't. It's more like plumbers putty in tape form. Butyl tape will stretch until it's very thin. Putty tape won't.

Butyl tape (not caulk) is a bedding compound, it's meant to be squished between things to seal them. Dicor is a company which makes many products, including both butyl tape and lap sealant. One can only assume you're talking about their lap sealant, which is popular around here. The name says it all - it's a lap sealant, it's meant to overlap a seam. And, it's self-leveling, so is not suitable for anything on the sidewalls, it will just run and make a mess.

You bed a roof vent in butyl tape, screw it down, wait a day or two for it to squish out the edges, remove what comes out, then cover the seams and screws with lap sealant. Similar for sidewall stuff, except you caulk the edges - I use Sikaflex 221, which I prefer to the Geocel Proflex often mentioned around here, for a number of reasons.

I do not know if each tape Winnebago uses has a different composition but I do know the butyl tape they recomend for windows is pretty darn good. Winnebago does not recommend using silicone on the edges and the tape doesn't squeeze out.

The windows that were replaced due to leaks between the glass look terrible. I bought the tape Winnie uses and I'll be resealing the frames to the walls.

Butyl tape is sticky and seals better than putty tape.

Thanks everyone. Do you have a source that you like for the "good" butyl tape?