The following code:
from mathutils import Vector
vector_a = Vector((0, 1, 2))
vector_a[0] = "Hello World"
incorrectly reports a wrong type for the index:
TypeError: vector[index] = x: index argument not a number
but it's actually the value that is wrong.
If you try a proper value, but bad type as index, the following error is reported (correct):
vector_a[...] = 123
TypeError: vector indices must be integers, not ellipsis