For the moment, I haven't uploaded any of my vectors as vectors - only as JPEGs which all the sites except iStock allow.
I agree with you that I'm not willing to let my vectors go for subscription prices - which would be the way DT, SS and some other sites work - but I have no trouble selling the JPEGs from my vectors for subs prices, and I'd have to give that up if I were exclusive for vectors at iStock. As I have a small number of vectors, I'd only make 25% on those if I were to be exclusive, and given the sales volume I've seen in the last year or so, for me, it just doesn't make sense.
I'm not sure about CanStock and StockFresh - whether their subscriptions include vectors or not, but they do offer higher prices for Vectors than the JPEGs