Just thought I'd put my oar in, I'm also considering a paludarium conversion with an old fish tank, but mine is 70 litres rather than gallons! I've never kept vamps although I've had RCCs before, but that's exceptionally large for 3 crabs as far as I can tell from what I've read.
In one forum I've seen someone has 50 vamps in 65 gallons. (Vampire Crabs - Gecko Resource Forums
Up to you, but if you're looking for 3 I'd be tempted to keep the big tank and use it for something larger scale, and get a little tank for your vamps.
Having said that, if you're breeding them, you'd be able to keep all the offspring! I think they give birth to about 10 live crabs every 6 months.
I've seen lots of forum posts saying they're not difficult to breed, so good luck, let me know how it goes