If the size 9 sandals were too big, I would go with the 9 in Vionic shoes. I wear a 9.5 and go down to a 9 in their sandals (every single pair of tide and I have a lot of them) but in the "bella" style, the 9 is too tight across the strap (so I don't buy those). For shoes, (sneaker and boots), I go with a 10.
I think you are probably OK with a 9, unless you have a wide foot, then go with the wide size 9.
I wouldn't size down in Vionic shoes, only in the sandals and they usually recommend if you are in between whole sizes, to size down in the sandal presentations.
......You look like I need a drink.....