Vault definitely does not need to proc marks, and it's not necessary on Blinding Assault, which deals good damage. E-auto-Q is a lot of burst that leaves the enemy carry only able to fight back with skills.
Making it pass through enemies is just going to make it an old Ezreal Essence Flux, albeit slightly weaker. If you're going to do either of those changes, then you have to remove the blind or at least make it more difficult to apply.
I'm assuming this is counter play. Considering the ability is essentially worth 2.5 auto attacks.
The second cast in an execution. Optimally (to use the second cast), you'll want to use this ability to get in range on a low health target in order to burst them down with the second cast. For what it is, it's versatile.
Also, the burst is balanced in the sense that it doesn't increase from Quinn, it's just easier to get all of your damage onto a target because everything becomes targeted.
The thing about her ultimate is that it's not something you cast at the start of a fight. You use it more near the end when you need it to escape and catch, as you've already mentioned. Another use, though not as efficient due to the high cooldown, is abusing the 80% AS to split push turrets down really fast.
If it were to pass through units (it would actually be like a caitlyn Q) then id suggest it only blinds the first target it.
Im not against being stunned mid-way throught the ability.
But im looking for consistency. With other movement abilities, the stun does not affect the movement because it has happened while the movement is occuring. It should be the same for vault. Stunned before the ability, or after, normal. Stunned during it, its applied after you have vaulted off.
Still counter-play, just more consistent with similar movement abilities (ie, stunned mid-way through flash, or a Sejuani charge, etc etc)
My point if, if your playing with quinn, against lets say, Riven, where not even E will be enough to escape, having the option to double tap R within quick succession to quickly finish her off is better then being forced to R, run away, wait for CD's and then MAYBE jump back in for the kill.
I know these little things with her ult. They are awesome. I just feel the restriction that you tag-team to Valor and then immediately to Quinn within a couple of seconds a bit to restrictive.
As it is now, if you want to execute quickly cause your both low, You want to be using R to escape, wait for CD's and then jump back in from somewhere else. In that time a TF or Noc or Panth ult can just ruin that. Id like the option to get the execute right then and their.
Kinda like how when talon ults, he can press R or auto-attack immediately to call the daggers back in.