I think it depends on a couple of things. The various vaporizers have different capacities. I have a Vapor Cannon, and the standard wand gauge for that is larger than that of the Hot Box. I have seen videos of the capacity of the Volcano, and it looks to be even larger than the Cannon.
Still, I agree that it is very easy to get carried away.
I think that less can be used, but it's necessary to modify your habits. Thanks to the fact that it rarely leaves you feeling like crap for the rest of the day, as is often the case with smoking, the need to refresh your high throughout the day is eliminated. If you can discipline yourself to walk away from your vaporizer after a quick smoke, it's easier to conserve. My problem was that I was as attracted to the process of vaporizing as I was to being high. I went through my weed probably twice as fast as I should have. With a conscious effort to relax after each wand and enjoy being stoned for a while, letting it take full effect, my usage has gone back to a very reasonable level for somebody that starts in the morning.
I don't know if that applies to you.