I don't know anything about
FW shrimp, so this may be a silly question. Is the vampire shrimp in any way aggressive, or is the name misleading? Your other tankmates are very peaceful, so I doubt the problem lies there.
It's possible that your corys aren't too fond of the bright lighting. Perhaps you could stack up a few pieces of slate and build some shelf-like rock formations that would shade them from the glare.
My corys love a few clumps of Java moss - they play inside it and rest on top of it at night - they tend to congregate beneath it during the day when the light is on.
Other than that, it may just take more time for your corys to feel comfortable enough to venture out in the open.