Hi 3v0,
by "not vacant" I mean that the named via belonging to a net can't be used for any other connection.
Placing any desired amount of vias on a PCB also means that there is no schematic loaded. Using this method you can certainly create masses of vias also by the copy and paste method.
You can add vias to a board created from a schematic with the schematic loaded. The above is not true.
If you load the schematic after the changes on the PCB are done the ERC will fail, and no more forward- backward annotation will be performed. Even changing a part with the same function and value, but different size, will not change both, schematic and PCB.
It is a very bad idea to close the schematic with the board open. I added the vias to the board (not schematic) with both open. ERC does not care if you add vias using the board editor. Vias are not components. You are making this much harder then it needs to be.