I question why you would want to fly so slow, and to the point where you think you need to use vortex generators to increase safety margins. You're asking for trouble if you want to leave the ground that close to stall - especially towing a glider. Keep yourself safely above stall before leaving the ground please, then you won't need those silly VGs.

If you are "often towing with a 15 - 20kt gusty DIRECT crosswind", you are a brave bunch. I don't know many folks that would consider doing that at all, let alone with a glider in tow. Be safe.