Have vinyl clad metal entry door with a very large bubble/blister, how does one remove the rest of the vinyl, prep & paint the door?
I would suggest you not remove the remainder of the vinyl.
My tendency toward a solution would be to prime the door. Apply an exterior grade filler, prime the filler and paint. While this will be slightly more involved than written, it is what I would do to affect a repair.
First, ask your local paint store guy, for a bonding type primer. Stay away from Kilz as a fix-all. Prime the entire door so your brush strokes will not be visible in the finish product.
Second, smooth on an exterior grade spackling. Personally I do not find the "Shrink-Free" types easy to use, but be sure it is an exterior use product.
Third, sand the area, check if a second application is necessary for a uniform look.
Last, if need be sand again, prime again and then apply two coats of finish.