Ralphy,
debris\vx = -(debris\vx - vcmy) + vcmy
is just the same as
debris\vx = -debris\vx
Obviously, I haven't seen all your code here but all you seem to be acheiving is making the debris and the object bounce off each other with the reverse of their current velocity. The Mass of the debris or other should make no difference?
If it seems to work for you .....
debris\vx = -(debris\vx - vcmy) + vcmy
is just the same as
debris\vx = -debris\vx
Obviously, I haven't seen all your code here but all you seem to be acheiving is making the debris and the object bounce off each other with the reverse of their current velocity. The Mass of the debris or other should make no difference?
If it seems to work for you .....