I wear a 9.5 also and ordered the 10, but the toe platform area stuck out 3/4" past my big toe ( I have long toes), so I ordered the 9. It is a much better fit and everything else is adjustable. I returned the size 10 and am happy with these sandals!
I usually order 7-1/2 W shoe. I have a wide foot so ordered a 7W (down 1/2 a size) and because the arch was so high my foot had way too much room and my foot rolled to the right and my walk was very unstable. If you are familiar to Vionic and the high arch is what you need my advice would be to go down to the 9 and order a narrow as the shoe is totally adjustable for width anyway.
I would order a 9. I'm a 9.5 shoe (10m in running shoes because the toe box, 9m when open toe shoes). I actually bought these in 8 because the 9 had to much space over the edge. I have flat feet, but skinny feet and skinny ankles but need the width in the toe area due to surgery and flat feet. Unless you have a very wide or bigger foot in general (over average size), a size 10 in this shoe would be to big probably. If you have a smaller foot (in terms of measuring around your foot), i would size down.