Hi Aaron,
What is the difference between Vitreolina and Eulima?  I'm sure no-one knows!
Well, maybe someone does, but seriously, the family Eulimidae is rather poorly
understood.  There are about 70 different genera described, and almost all of
them are slender little glossy white shells, mostly under 15 millimeters in
length, and many under 5 millimeters.  The experts can't even seem to agree on
how many families this group should represent.  To anyone trying to identify
these little beasties, good luck!
As for your other question, many species of Eulimidae have a normally curved
spire.  Picture a small, smooth, glossy white Terebra (auger shell).  Picture
it bent into a smooth crescent shape.  That's what they look like.  Usually
the bend is in one plane only, but in some forms after the spire curves in one
direction for a while, it alters course and curves in a different direction.
I assume this is what is meant by "shell bent once" and "shell bent twice".
Paul M.