Malachia

I think that "one breath" is just the amount of investiture each human is born with on Nalthis, there is no indication anywhere that it is a unit. The Ars Arcanum even states that the numbers needed for each Hightening are rough estimates. The fact that they needs one breath a week only indicates that they needs a certain amount of investiture consumed each week. I suspect the amount of Stormlight that can be stored in a single mark (not so much) is at least 1 breath, if not more. Whats to say that each highstorm Vasher is in he doesn't get supercharged enough to last him weeks? Also, I don't think he needs to store the Stormlight in his body like a surgebinder would. I don't really know enough about the anatomy of Returned but I would guess he knows how to store Stormlight as Breath, I think he just can't use it for awakening on Roshar (different Shards=different magics). I'm pretty confident though that even if we think he gets charged like gems every storm and he had bundles of Stormlight stored in his body he could still make himself look normal, much like he could on Nalthis.

As far as I can tell, Stormlight is a lot more efficient than Breath. If you look at the comparison between Shardblades and Nightblood which has been going on for a while around here you can see some very interesting points. Brandon himself said that on Roshar Nightblood is practically a Shardblade but stronger (cant find the link to the source right now, sorry), this is something I thought of even before I read him say it, since when Vasher used Nightblood at the end of Warbreaker, he could slice through anything, just like a Shardblade. Another interesting point is that at the end of WoR Syl tells Kaladin that the Honorblades feed on their wielders Stormlight and that it takes a LOT of it. Just like Nightblood consumes Breath. This leads me to believe that Nightblood is more like an Honorblade than a Shardblade, but that's beside the point. If Brandon said Nightblood is actually an Honorblade I think we can assume that an Honorblade and Nightblood would consume the same amount of investiture. BUT if you look at the scene where Vasher draws Nightblood, he goes through thousands of breaths in a matter of minutes, if not less. Compared to Szeth that seems a bit extreme doesn't it? I mean, Szeth's blade took a big chunk of his Stormlight but its not like merely holding it caused him to be drained of all light in a matter of minutes. My conclusion is that Stormlight is just more efficient. I would guess the last Highstorm would be able to hold Vasher up until the end of the weeping easily. P.S. First post on these forums, be gentle XD