When the rocket motor is activated it will reduce the weight of the balloon payload, causing the balloon to rise. As rockets tend to accelerate very slowly to start with, will the Vulture 2 craft acelerate quickly enough to escape the launch rail at all?
I suggest that instead of a launch rail, Vulture 2 is attached to the payload spar with slow-burning fuse, ignited by the rocket motor (with appropriately fire-proof connections to the craft). This will achieve separation of the components irrespective of relative acceleration.
Otherwise my guess is that the balloon will accelerate as quickly upwards ar Vulture 2 does, so you might as well attach the rocket motor to the payload spar and release Vulture 2 when it has ceased firing. Vulture 2 could then be a glider with better aerodynamics and less weight.