So you're in a bind because a key person has left. Other team members do many of the things that the key person did. You're looking to hire another person in the same role that you can't afford to lose.

Wouldn't it make sense to decouple some of the requirements? For instance, there's pretty good separation between front and back end. The person only needs a conceptual understanding of the other side and detailed understanding of the protocol to be used. I use both Vue and many backend tech, but my understanding is that I use Vue with a REST api and reactive rendering. The api design could be iterated by members from both groups.