As above, do they pull the same amount of cable.?
No
Will I die if I use v brake levers.?
You won't die, but your brakes will feel very hard and have less modulation than with a properly matched lever. A V lever pulls much more cable than a canti brake lever.
They'll just be a bit crap - hard and little power
I expect that with some bodging the V Brake lever will be OK for the canti but not the other way around? As long as you have and adjustable reach then I am sure you can get them to a usable position 🙂
You can get inline bar levers for cyclocross which plug into the STI setup?
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Had the misfortune to ride such a set up many years back(before V-brakes existed, Kona High Command levers were mis-specced during production and ended up with a similar pull ratio to V-brakes). Superb solid feel to the lever, virtually no stopping power whatsoever though, the only chance of locking a wheel was on slippery mud, just when you didn't want it to.