That's what I think too - adding a Bigsby opens the tone up - something a Vibrate doesn't do.
I don't know if it sounds different, but I guess that would make sense. I just don't like a wonky mounting method just for consumer indecision and convenience. It's lame.
My theory on this is that hollow and semi-hollow guitars often lack the strong sustain that many solid bodies have. With vibrato provided by fingers or a whammy, it tends to prolong the note(s) longer, creating a little more sustain. But that is just a theory.
I lump semi-hollows and solidbodies together, because they are more like each other to me. You may be right about sustain, but I think timbre is way more complicated than that, so I stick to just evaluating if I like a sound or not. It's not always more or less sustain, or something I can pin down that easy.