I own a Mission Workshop Vandal, but it now gets used only for air travel or non-bike specific tasks (for now). I know that when I replace my mountain bike, it will probably become my go-to bag for bikepacking again until I get a bob trailer.

However, having ridden thousands of miles with it on, I can say that when I built up my Vaya, I went with the Salsa rack and a pair of Arkiel panniers - which I do not regret at all.
The bags do hold less than my Vandal, but they're not on my back. This is a big deal when you're pushing 20 miles in 90+ degree tropical weather conditions. I'm still soaked after the ride to work and home, but I'm not getting significant skin irritation where the bag sits anymore and, much more importantly, I'm not overheating in the sun.

I think backpacks definitely have their place, but when it comes to commuting, I'm finding that as I break over the 600 miles a month mark, that place is firmly in the short, limited haul capacity.