I'm working on the Girl Friday
cardigan right now. I'm not to the button bands yet (my first time doing them!), but they are picked up and worked in one piece with the collar. And it looks like they're vertical. You BO three stitches going around once, then CO three stitches coming back. It's a reasonably snug sweater on most people and the vertical buttonholes don't seem to be an issue.
Edited to ask:
Since I've never done button bands/buttonholes before, isn't it actually possible to do horizontal buttonholes even if you pick up the button band to work it? I'm assuming you' could just bind off, then join a small ball of yarn between each hole, much like you'd BO and join to work the two sides of a neckline or to make the holes for the legs of a dog sweater. Granted, I also assume this would be a pain in the butt with all those little balls of yarn, but it would only be for a few rows. Is my logic correct?