Hi,
you are right, it uses almost same tables, because ENYcreations based their code on one of the previous VM Invoice version at certain moment.
From that reason, the installation of both at same time may be complicated if not impossible.
Anyway, solving such problem is out of range of this support.
My recommendation is to archive the existing tables, remove the ENYcreations component and install VM Invoice.
As the invoice data self are loaded from VM you will also be able to generate invoices for the old orders. (just - logically - in appearance these may differ from invoices previously generated by ENYcreations in some details; but the items and sums should be correct)