The problem might be that with an upper body masked out, it would bounce around quite a lot, because there is no spine animation to compensate for the bouncing of the hips, so the motion of the hips carries on rigidly to the entire upper body. Solving VRIK on top of that would indeed give you pretty strange results probably.
I've used MovementAnimsetPro and found that the upper body motion looks pretty good with VRIK without any masking, but you can try to stabilize the upper body with AimIK. It would solve before VRIK and help keep the upper body in place a bit more. Add just the spine bones and assign the last spine bone as the "Aim Transform", move the target forward and parent it to the root of the character or to the head target, play around with the weights a bit to tweak it to your liking.