It is important to understand why you are seeing Night March/Vespiquen decks.
When did you quit playing? I can hazard a guess based on what you've said but there are exceptions that could prove false everything I can deduce.
Part of why I ask is because the Expanded Format. You would have quite while it was brand new, and it sounds like you're discussing a deck more often seen in Standard (Night March/Vespiquen) but actually wanting a deck for Expanded, since
Flareon [Plasma] is the only one that really makes sense with Night March, while I'm not sure if any
Leafeon other than
Leafeon [Plasma] makes sense in any deck right now. XD
So if you are talking about Expanded play, then you can run
Flareon [Plasma] with Night March. Most run
Flareon [Plasma] with
Vespiquen (just to be clear, I mean
XY: Ancient Origins 10/98) because they use the exact same tactic and seem to compliment each other well.
Vespiquen in Night March is more a necessity for Standard than anything else; Night March needed another attacker and
Mew (
XY: Fates Collide 29/124) is so new that it still isn't tournament legal.
Vespiquen became one of the man options because it gave the deck an Evolved attacker (to get through certain anti-Basic effects), has a free Retreat Cost, and hits
Seismitoad-EX especially hard. Since Quaking Punch is still difficult for a Trainer intensive deck like Night March to deal with and
Seismitoad-EX is Grass Weak, even if your opponent locks you down fast and your discard pile is small,
Vespiquen might be up to the job.
So if you want to run Night March with
Flareon [Plasma] in Expanded, go ahead. I would try to get the cards you need to build the decks separately as well though, because I believe the established versions are better suited to the current foramt.