Ok, I get your point about select not being loved by the user. Let's iterate on the replace-like name that's proposed.
Linguistically, replacing(with:where:) is missing an object to form a verb phrase. It's implicitly saying it's replacing elements with elements from another vector, so we can make it clearer by naming it replacingElements(withElementsOf:selectedBy:). In the following use case, it's a bit verbose but the meaning is immediately clear to me.
let mask: Int8.Vector3.Mask = [true, false, true]
let v1: Int8.Vector3 = [1, 2, 3]
let v2: Int8.Vector3 = [4, 5, 6]
v1.replacingElements(withElementsOf: v2, selectedBy: mask)