I am in an open evangelical church (per the ACNY entry - clergy from Trinity Bristol, Ridley and the place in North London whose name escapes me, and St Mellitus). My vicar recently referred to a partner church as reformed (inferring that we weren't). I don't see a lot of difference between my church and Presbyterian, apart from active liturgy (and general vibrancy, presence of young people, and full-on expression of faith).
If knowledgeable people here can define reformed vs open evangelical (in non-disparaging terms), I would be extremely grateful.
They aren't necessarily opposites, and the word reformed can be used in different ways.
Essentially open evangelicals tend to be middle of the road evangelical. That is, they see the development of a personal faith relationship with christ as the key for salvation, and they therefore tend to emphasize outreach carrying that message. (Of course, lots of other congregations believe much the same an don't call themselves evangelical, but there you go.) Open evangelicals don't tend to be the really nutty conservative types you see at times though - they would contrast with the sort of people you see insisting on creationism as opposed to evolution, and they are generally more socially liberal. Many might consider their congregations inclusive.
Being Reformed, or sometimes reformed, can just refer to an emphasis on the lordship of the person of Jesus Christ. So that can be similar to open evangelical in that there will be that kind of emphasis in outreach. It doesn't necessarily mean low church, or happy-clappy, and it could even be very conservative theologically or socially or remain very liturgical. When used that way, it is a very very broad term. An open evangelical parish would probably be comfortable calling itself reformed in this sense.
However, very often it also implies being influenced, perhaps quite strongly, by Calvinism. If it uses a capital "R" outside of a proper name, that is most likely the case. That can be strong or weak, but I think it would be unusual to see an open evangelical parish that was very Calvinist.
Neither has anything to do with vibrancy, or whether there are many young people, or the full expression of faith.