Please support Vandham. If for nothing else for how he taught Rex how to topple with Anchor Shot (way cooler than the Monado Purge Alvis teaches during his stint as a temp).
Alvis fights alongside you for one battle, where he isn't directly controllable, only has two arts, and probably could have just watched from the sidelines and nothing would have changed. Vandham on the other hand needs to be playable for his role in the story to work, and so he actually earns his temporary spot in the party.
He's also a much more fully realized party member. He gets the full five arts, he gets his own Affinity Chart with his own skills, and while he can't awaken Blades, him only having one Blade stands out less since Tora also only has α at this point. The only temporary party member who's more fully implemented than Vandham is is Lao, but he's not around anymore, so Vandham's second place on that front is no. 1 in this comparison.
Minor side note, but did you know the game has six Pouch Expansion Kits for you to find. That means you can use a Kit on Vandham, not realizing what'll happen to him, and still be able to expand the pouches of all five permanent members.