I have had some dealings with the vantage over the last few years. I have always had large helmets, and even with other Smith helmets, I am a large.. but for some weird reason, I am a medium in the Vantage. The large felt good, just a little wobble on top of the head, but once I tried the medium I realized how the helmet was supposed to fit, and it kept my ears warmer since it hugged them closer. Not sure why exactly, but if you are right on the cusp, I would go with the small. For reference, I am 6'3" and have been told I have a big head my whole life...
I'll second the motion to buy from backcountry.com, they have the best return policy out there.