Honestly Leafeon and Vaporeon both have pros and cons.
Type advantage wise, Leafeon would add an unfortunate disadvantage as far as bug type moves considering that Krook and Beheeyem both have weaknesses, but bug types won't be much of a threat since you have already beaten the bug-type gym leader and they won't be too common. Also Typhlosion shouldn't have much problem with its fire type moves. Typhlosion has some decent synergy with Leafeon, boosting its fire type attacks (with flash fire) when hit by a fire attack which should be fairly easy to lure in. Typhlosion also won't have any problem tanking those ice type moves and hitting back hard. Poison honestly isn't much of a threat to either Vaporeon or Leafeon.
Vaporeon would add two weaknesses, grass and electric. The only other poke weak to grass is Krook, but Typhlosion could easy take it down. If you added Vap then it would be the only poke weak to electric, and Krook would easily destroy electrics.
Leafeon has a fairly shallow moveset, learning Swords Dance and Leaf Blade leveling up, and even sunny day to help out typhlosion and for the chlorophyll boost. Leafeon could also hit hard with Last Resort but those are some of the best moves it could learn via level up. You could do lots of breeding but that wouldn't help much.
Vaporeon has a decent moveset in Hydro Pump/Muddy Water, Signal Beam/Aurora Beam, Shadow Ball, and maybe Rain Dance or Wish; the latter available through breeding.
Both seem like fair options for your team, but in my opinion, Leafeon seems like the way to go balancing out physical and special attackers (I assume). Also Leafeon can pack a hard punch. You shouldn't underestimate Vaporeon's bulk though, it can tank a lot of (special) hits and has some decent special attack on its own.
I hope I could help~