I had to glue my vulcanite several years ago and think I did use superglue (it is soluble in acetone if it needs removing later - if the vulcanite is being removed anyway at that time, some acetone is not going to harm it, you just need to keep it out of the camera body). My vulcanite had lifted from the shell of the body and this left a space which could be exploited by putting a few drops into the space between. If you have a gap too then I think you need some glue that will fill it so a highly liquid glue (like PVA glue) is not going to work. Also a slightly thicker, semi liquid glue will not flow into areas you do not want it. But it depends upon the size of the area - mine was quite small. The attraction of superglue is that it flows but with care can be controlled, sets hard and has a small nozzle to control the positioning and quantity. But there are others like this available in hardware stores. One that comes to mind in Australia (my home) is called Tarzans Grip. It may be available in other countries too. Another option is I found some craft glue which smells of some kind of hydrocarbon solvent (much like Tarzans Grip) and is semi liquid and flowing. That would work as well. Also model stores (i.e. those that sell model planes etc) have a similar plastic glue with the same smell which should work too. It comes in a small bottle with a long very thin nozzle that would be perfect for the job.
Apart from a camera with problematic vulcanite, I also had an old 90mm Elmar f4 which has vulcanite around the base of the lens and a small part of it had broken away and been lost. It was only a tiny piece so in that case I used some black bituminous roofing compound (again from the hardware store) to apply a small dob of the compound which I reworked to look like the grain on the vulcanite. It took a week or two to set but when it did it was near invisible being made from basically the same material as vulcanite. Keep this in mind if you do lose some vulcanite.
