TacitBlue64

It appears there is at least one temporary fix I've read about and can confirm works. If you send the craft out to launch first, you can once again merge other vessels with it if you revert back to the vehicle assembly/spaceplane hangar, however it may have to be done each time you load another vessel, so it is time consuming.

Well at least it's a known problem then. Hopefully the patch will get released soon.

For some reason when I try to merge one file into another in the editor, nothing happens. The "merge" button responds to being clicked and released, but that's it. This happens in both the SPH and VAB, and switching to another file doesn't change this. I'm using version 1.10.