The nice thing about the Vandal (or it's slightly smaller cousin, the Rambler, which I own) is that while it EXPANDS to fit a ridiculous load, when you don't have that much stuff you can use it un-expanded and it's fairly compact. Not a lot of fabric and whatnot flapping around. But sure, there are times when I don't have a lot of stuff and don't want to lug the packpack around, and I use my old Timbuk2 messenger.

When you carry a lot of stuff/heavy stuff, the difference between a one-shoulder messenger bag and a backpack is almost immeasurable. I love having an optional waist strap for heavy loads, too.