Everyone's feet are different. I love my many pairs of Vionics, but absolutely can't walk in Birkenstocks- they aren't shaped like my foot. I discovered Vionics when I had plantar fasciitis and felt instant comfort when I wore my first pair of Vionics (then called Orthaheel). My arches are high, and that footbed just fits into my arch and supports it. If your arches are kind of low, the Vionic arch might not be so comfortable. Today is a good time to give them a try, with the 3 EZ pays, because you're only venturing 1/3 of the price to give them a try. As far as sizing, I'm a 7 1/2 and wear the size 7 Vionic with some room to spare. Oh, and for what it's worth, I still have all of my original pairs, which are about 4 years old now. So they have held up just fine.
"Breathe in, breathe out, move on." Jimmy Buffett