In no way is ether theory like the quantum vacuum. The quantum vacuum is still a vacuum, devoid of real photons and matter. Ether was thought of a medium that propagated the electromagnetic wave and comes with all the consequences of a physical medium. Ideas like ether drag and such are examples of experimentally debunked properties of said ether. The fact that Tesla would work assuming an ether strongly suggests that he was still caught up in the thinking of very early electromagnetics. It should not be any surprise then that he would seriously research topics and ideas that would prove unfeasible and ridiculous by modern standards simply because we have the hindsight of many many years of research and experimentation. Carnot subscribed to the idea of calorics as being massless particles that mediated the transfer of heat. The idea is of course very wrong but he was still able to produce his ideal Carnot engine that is still a very fundamental building block of modern thermodynamic theory. You can find many ideas and theories from many of history's prominent scientists that are wrong by modern theory and experiments. Why should Tesla be any different?