Thanks guys! 
My guess from your pics is that you actually have the Geosesarma sp. "blue" and not the standard vampire crab (G. biciolor) since it doesn't look like yours have an orange or yellow blotch on their backs.
And that's where they'll stay! LOLThey have their favorite places to hide.
And that's where they'll stay! LOL
I've never seen all of mine out at once. I searched real hard once and spotted six - the other 4 were who knows where.
How many did you buy? Have you lost any yet from sudden death? I lost one 12 hours after unpacking them, I had just taken his pic and when I next checked on him he was face down in the little pool. I found another one dead last week with no injuries, the seller told me it was not uncommon to loose 20-30% when they are acclimating to captivity. I hate that but I guess if you can get them breeding the babies are said to be hardy and not as stressed out as the WC imports.
The crabs may not be hungry when you fed them so slowing the crickets down might not be worth the effort, they'll warm up quick again but the crabs have no problem catching the small ones, mine are lightening fast when they want to be. I put 6-12 small crickets in their tank - you can dust with calcium on occasion so they get some extra calcium in their diet. I also put a few broken up cuttle fish bone pieces in the tank, they will scrape on it and eat the tiny grains. You can also put wet food on it (defrosted bloodworms) and they may glean a bit of calcium from the wet worms sitting on it.
I've never had that happen to any of my terrariums. Any damage the crickets may possibly do is negligible, they aren't in the tank long enough. I keep all my feeders well fed so they are not starving mad when they go into the enclosure and only add as many feeders as you think they will eat. I gut load with high protein ground dry cat food and have a bowl of watersorb.com water crystals for them to hydrate on. This feed seems to reduce incidence of cannibalism in the cricket bin as well.but I dont' want the crickets eating my plants