I think waiting for VM is not the best approach. In the end, it will be changed through community effort. Anything that is done because of this thread will surely be beneficial.
As I see it, the VM side of things is not so complicated. The return to VM will be the same, the only difference will be that the return code from, for example, Authorize should reflect the part-payment, not completed payment. So then, instead of the confirmation page, you'd see a different page (with the payment link again).
At present, we've only been working with unsuccessful / success return codes (usually as Confirmed / Pending), so there will need to be the addition of another I presume.