Hi and welcome to the Meteor forums.
Quite the task you’ve got there!
Now, there is no easy way to achieve what you want, but equally this is an amazing opportunity to learn a few extremely useful things. In what follows, I don’t outline a recipe, but a potential course of action which I would take if I had the same problem.
First, approach it as an architectural challenge, not a technical one. The latter is easy once you figured out the former. Because the routers for each UI library are specialised, you won’t be able to easily (at all?) make both Angular and Vue coexist – others with more knowledge could chip in.
Second, to make the client codebase more manageable, and futureproof (cause the world of frontend frameworks changes by the month), I suggest a micro-frontend approach. See if it’s feasible at all to take something like meta-router and make it work with Meteor’s reactive modules, like Tracker, ReactiveVar, Minimongo, etc, as these ones, I’d expect, could be used in both Angular and Vue.
I suggest taking a good luck at Vulcan’s architecture and reading about its philosophy and patterns – you don’t necessarily need to use Vulcan itself. There’s almost no limit to the modularity which you can achieve with Meteor and the Vulcan architecture. And modularity is your way out of hell.