i have bent a few VM bows , some more successfully than others. here are my observations:
I steam for at least an hour, more if thicker, and i try to trap steam around the area with a tinfoil "tent" or a section of downspout.
When steaming rigid parts of the bow (tips) - the bend seems permanent,but while steaming the bending parts, the bend comes out almost completely. I think this is true of all woods.
I have put a 45 degree bend in a VM limb tip, but have had less success with bending the handle.
I don't know if you can over steam it, but i'm guessing your problems arise from too little steaming, or a setup that does not direct the steam well.
good luck!