When I saw the first half of the question, I was expecting it to say, "Would Vauxhall have sounded a lot different if Mick Ronson had lived long enough to produce it?" Now the idea of him producing (and possibly writing) the music to go with some of Morrissey's best ever lyrics is an interesting thought.
As for the actual question, I don't think Gary or Spencer would have improved the album. They returned for the Moon River session, which was deemed a disaster, and then promptly departed again. Spencer's inclusion was sensible for Southpaw Grammar due to the rock style but, imo, he and Gary leaving was of no great consequence.
It's amusing that so many people talk about "the band" as if the 4 of them were together with Moz for years. In actual fact Alain, Boz, Gary and Spencer played together for a relatively short period of time.