Thanks for the excellent review, Will. It's great to see another manufacturer moving further in the direction of ultralight, high quality gear.
In discussions of bag length, I don't think I've ever seen acknowledgment of the importance of foot length, and ankle extension. A person with a longer foot, and a looser ankle, will have an overall body length considerably longer, when lying on their back, then someone with a shorter foot or a "stiffer" ankle. So in a shorter bag, a longer-footed person will more easily have the feeling that the bag is too short, as they feel their toes pressing agains the foot of the bag, and their head against the hood. I've never done the research, but I'll bet the difference between a stiff-ankled size 9 foot and a loose ankled size 11 foot could be easily two inches.
Another thing to note, though I haven't seen the Vapor, is that the Marmot helium is, as I remember, generously cut (i.e, large cross-sectional diameter), and the vapor looks more compact, judging from the photos. So the average down density per square inch of bag surface, which is, I think, more important than the total down in a bag, might favor the Vapor.