Vulcaseal is no good at all. It merely possesses an excellent brand recall in the same light as Silver Swan, Colgate, Xerox, Kleenex, etc...

You need to use silicone based sealant. Not just any ordinary silicone sealant for the sink but make very sure its NON ACIDIC.

As for the "dugtong", it could be the previous installer failed to provide adequate overlapping of GI sheets, which will lead to immediate leaks if a sheet's overlap suddenly rises when it becomes unfastened by the wind or loose nailer.