I think that it would last more than 6 months in a vacuum pack - I tend to get more than 6 months in a regular tin, even being opened periodically.
I would say that it depends more on how green it is. If it's completely green then it will probably be diminished at this point but I would still think it *somewhat* enjoyable - I've had green oolongs a couple years old that were still plenty nice from being vacuum packed.
The "traditional roast" isn't going to be as fragrant, and I've recently found that it can sometimes take a lot of heat to really unlock the best qualities. Until I found this I thought of these as very lackluster and boring - a simple "honey" taste without much dimension and little aroma. The very high heat brewing, however, brought out a nice woody/fruity complexity with light notes of berry and spice. It's a little hard to describe. These aren't as easy to find these days because it's a fair amount of work that's probably not worth it when it's the very green ones that are most popular.
A heavy roast isn't going to be fragrant but shouldn't be degraded much at all. These can also take quite a bit of heat to get the most out of, but should still have plenty of flavor anyway. These are even less common, probably because it's neither popular nor traditional, but that's just a guess.
Of course it's also possible that you just got crappy tea
My bet would be on the second, though. Try brewing it as hot as you can get it. Use yixing if you can, otherwise the thickest walled brewing vessel you have. It would really be best to brew it gongfu style - so the hydrated leaf fills the vessel. Pre-heat it more than once, so that it's pre-heated as hot as it will get and minimize the time that it has to cool off (meaning get the leaf and water in fairly quickly, rather than letting it sit empty while you do other things).