For example if I had a Vulpix holding a everstone, and then I used a fire stone on the Vulpix whilse its still holding the everstone will it still evolve?
A Pokemon will still evolve when the correct evolutionary stone is used, regardless of whether it is holding an everstone.
Source: I just tried it with my Pansear
>Another stone, the Everstone, is different in that radiates a type of energy that prevents evolution in the holders; however, if a Pokémon holding the Everstone has an evolutionary stone used on it, it will still evolve.
Source: Bulbapedia/ Evolutionary Stone