Unfortunately, as cool of an idea as this is, it won't work for vampire crabs.
I don't have experience with fiddler crabs, but according to the PawNation website, they could possibly work with this setup (along with some other species). Although I would still recommend researching any specific species that you're looking at getting, just to be sure.
The issue is that semi-aquatic doesn't just mean that the species can live in either the land or water, but that they need certain percentages of both environments to survive (For vampire crabs it's about 2/3 land to water). In fact, semi-aquatic is a good indicator that the animal is terrestrial, but likes water available to swim in occasionally.
This is the case with vampire crabs, the water in their terrarium is only used for drinking, molting, and breeding. Otherwise you will never see them in water (or at all really, they're incredibly skittish). The ideal vampire crab habitat is actually the same as a poison dart frog's, so you can follow a care sheet for them if needed.
Unfortunately vampire crabs are a pretty exotic species, where not much is known about their care. So when these crabs do make it into a pet store they are typically mislabeled, and placed in aquariums, where they will only survive for an extremely short period of time.
This is because the vampire crab's life-cycle starts out in the water, where they drop of their young. The young crabs will live in and around the water until they mature and their shells darken in color. Once vampire crabs are mature, they lose the ability to gather oxygen from water so easily, and while they still can for short periods of time (they still need to be able to in order to molt), the effort it takes for them to do so will kill them.
Here are some places to find more information about vampire crabs, unfortunately as I mentioned, they aren't documented very well, so information about them is scattered across forums.