When the new Vector and the top language selector box are one, I can't find a way to link an article to other articles about the same topic using Wikidata's "Add link" box.

"Add link" is a pretty important feature. I remember well how back in 2012 when Wikidata was first deployed, this was the editors' top wish, and the Wikidata developers quickly created it. It should stay usable with the new Vector.