Just got back from Supanova Sydney and got to ask a few questions of Brandon:
SPOILERS!!!!
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Q: Does Vasher have a different way of getting access to stormlight than everyone else? Given that he has no spren, no honorblade and he isn't a squire?
A: Yes. He can use stormlight to stay alive, but it doesn't let him Surgebind.
Q: Could Szeth get access to this method?
A: Yes
Q: And use it to fuel Nightblood?
A: Yes he could
Q: Can you tell me what that method is?
A: <Smile>