Actually, I've always thought of Vaporeon as being really feminine. It seems like a very graceful Water-type (a type often connected to elegance) with mermaid-like qualities and the ability to dissapear in the water. Not to say that Espeon feminine, because it certainly is, though it's really overused in fics, especially bcause when almost always paired with a dark, angsty Umbreon.
But I think the most underused Eeveelution is Flareon. It isn't the most elegant, a lot of people thinking it's a lot more large and bulky because of its mane and all.
Jolteon is certainly underused, though, but I think Flareon is used a lot less. *shrugs* Flareon also seems a bit harder to write for some reason.
Meh, it's your call, though, and if you've already decided...well, there isn't much I can do, then, is there?
~Psychic