Within Gutenberg itself, any library your plugin uses will be loaded, but these libraries are all quite small – Preact is 3KB, Vue is 20KB, React is 35KB, Polymer is also around 35KB. Even assuming a site has plugins installed that create blocks with every possible framework, this is still a fairly small addition. Of course, these files are rarely downloaded, as your browser is probably already caching a copy of them.

Regarding requests yes and no I'd say. But potentials of conflicts etc is probably the main issue.
Vue was far from the only framework being advocated for, though many of the discussions have been framed as a “Vue vs React” thing. Far from being that, I think the clearest thing to come from these discussions is that people want to be able to develop against the framework they’re most comfortable with, and I agree with that they should be able to. Core doesn’t need to force this decision upon them.
Not the only one but the one with almost majority behind it.