VS, I'm not sure if you've ever hit a whip before, but the difference is real. The Extreme is the perfect example of the difference because it does both bags and whip coming from the same mechanism. I love the bags, but the vapor is never as thick as when I hit the whip. Max explained it very well around here somewhere and I will try to do the same. Wish me luck.
When hitting the whip you get fantastic first and second hits...thick and tasty with a massive vapor to air ratio. As you continue to hit it after the first two hits (don't take the two hits as gospel here, just an example) the vapor gets less tasty and thinner with a lower vapor to air ratio.
Now take that same amount of inhaled vapor and put it into a bag. You're going to get consistent vapor all the way through that has a quality somewhere in the middle of the vapor hits. It is mixing the great first two hits with the lesser two to four hits that follow.
The vapor dome doesn't create true whip hits, as it simply fills a glass dome and allows you to draw it through a whip instead of the mouthpiece on the bag, thus getting a homogenized hit like with a bag. If the dome were to stay empty until the user started drawing the air through the herb into the dome, it would be a true whip hit.
Disclaimer: I don't own a Volcano and have never seen a vapor-dome. If I fucked this thing up, please let me know.