To everyone, Umbreon is whatever it is at the time you are reading this because of USAGE. Usage determines most tiering. Then, misfits are dropped into BL and Ubers. Obviously, people will try to use the best stuff in a competitive metagame(That is the whole point), so usage can dictate how stuff is playing out in the metagame better than having people theorymon it out, unless you're trying to get something banned (Salamence, Latias, etc., where a council was best to sit down and talk about it). Yanmega seems fine in BL, he's by far too broken in UU, but he's not used enough for OU. Umbreon is fine wherever it is. Really, there is no real need for change for either of these guys.
aRyuujin on LoL NA
aRyuujin.546 on SCII NA
Yanmega should stay BL. I've seen his sweeping capabilities as a lead in OU, and they are quite outstanding; however, it will never be good enough for OU, unless SR gets nerfed hard. The fact that it can only join the frey a maximum of Two times, is quite hard on him (Less if we factor in SS) not to mention the abundance of priority users (coughscizor) Will leave him sorely hurting. Very short repy, but it seems everyone else has made better points already.
As for Umbreon, it really depends on the metagame it's in. I mean, if we still had Salamence, Latias, and Garchomp, you can bet it'd be UU, but for now it seems to be OU. Perhaps, due to the flip-floppyness of it's nature, it should just take the middle ground and go BL.