My question is, why is an item specifically for Silvally being held by your Delphox? As for Terra, being it's the glitch field iirc Gen 1 mechanics apply and certain types become ??? type, i agree you should evolve your Vulpix but i don't think taking the tiers they are in should really be more of a concern. Your Furfrou assumably could act as a strong defensive wall? i'm not that well versed in matters like that but i will say that bringing an Electric type to a ground gym is limiting your options and taking up a slot which could go to a better choice, sure Emolga being part flying helps but Swoobat is also part flying and gets the benefit of being a Psychic type which is pretty strong in the glitch field, alternatively Jumpluff, also a Flying type but part grass too. Ideally i would choose either of them.
Keep in mind two of Terra's pokemon have an item that changes them to being ??? type on the glitch field and boost their defences i think by some means so you will have to either chip away at them with no super effective hits or forcefully switch them out so you can make her send them back out with their weaknesses to use.