Loosing vertical speed after landing on the ground sounds like the ground is doing it's job. Which speed exactly do you want to keep constant? Do you want a perfectly elastic bounce, or does your character loose horizontal speed as well? Do you maybe want to keep the airspeed and redirect it horizontally after the landing?
I don't know Velcrophysics but impulse is generally velocity*mass, so you would take the velocity difference you want to achieve and multiply it with the object's mass.