I usually shoot for mid grade (or Plus, in most areas).

Even though the VX only requires regular, I figure gas is a nasty dirty substance and the midgrade is probably just regular after it's all said and done.

Isn't regular cheaper not only because it's less octane but because it tends to be the bottom of the barrel or mixes of different grades?

There is no rhyme or reason to my theory and I'm not sure why I believe this, but that's what I do. Doesn't seem to be hurting the VX by doing so. However, since the gas price hike, I've switched to regular until the prices come back down