This is interesting really. I haven't heard much about vampires on this topic, but rather with bamboos. I'd guess that it would be similar between the two of these species as they are so similar. Both are filter feeding freshwater fish that initially live in brackish - though of course they come from different areas and are of a different genus.
So basically with bamboos, the adults don't enter brackish water. They breed in fresh, carry the eggs and the larvae are then carried down through the water current to brackish waters. As far as I've read, the adults themselves don't tolerate brackish water. I would think this would be similar with vampires. If that's the case, then I would say that they should not be kept in brackish waters. They have to migrate to freshwater for a reason as they grow after all (though admittedly that could be due to food and not salinity).
If they actually can that would be very interesting to hear, so I'll be following this thread just in case. My instinct would say no though.