And the Mojo standard library has been open source for over a year.

It’s all open source. Go check it out!

I feel as though we can retain understanding of those differences without using the name "Vec", which can be considered a poor choice for the reasons mentioned above (by C++ and Rust alike).

Array for a growable array

[T, ..N] for a fixed-length array

&[T] for a slice into either of these array types

Even forgetting the mathematical concept, I would argue that throwing the term "Vector" into the mix only makes things more complicated than they need to be, since sticking to "Array" homogenizes everything. For good reason, we don't call a resizable string a "Hobnob" in an attempt to disambiguate, so why call a resizable array a "Vector", which makes just as much sense?