This comment is pedantic but not relevant, the grandparent comment was right. If the vector is reallocated to a new location in memory (either because it uses malloc or because realloc can't grow the existing allocation and must allocate somewhere new), the references will be invalidated and the behavior of this program is undefined. Using realloc would not make this code correct.
The references also won't be invalidated if there's no need to reallocate, because the vector has enough space. Enumerating the cases in which this wouldn't cause UB does not change the fact that there are cases in which it would cause UB.
I'm sure it feels good to make comments like this, but it's a disservice to newer developers who may be misled by your comment about whether or not this code contains UB.
My comment acknowledges the issue mentioned by the parent comment and extends it with extra information out of my own curiosity, and which others may find useful or valuable too. Or not. I don't care.
There surely could exist std::vector implementation with std::realloc implementation under the hood where the issue described wouldn't always be reproducible? I think that MSVC actually uses this technique AFAIR.
The references also won't be invalidated if there's no need to reallocate, because the vector has enough space. Enumerating the cases in which this wouldn't cause UB does not change the fact that there are cases in which it would cause UB.
I'm sure it feels good to make comments like this, but it's a disservice to newer developers who may be misled by your comment about whether or not this code contains UB.