Short answer - store cold or as cool as possible and drink as fresh as possible. Only higher ABV beers have to potential to mature with age, and even then it becomes relative to your expectations.
Bottle conditioned beers (re-fermented in the bottle to produce carbonation) can also mature - such as bottle conditioned Belgian ales. But these too are often higher in ABV.
Highly hopped beers also have the potential to improve and mellow due to the antibacterial nature of them in preventing spoilage.
But the ABV is probably the best way to determine ageing potential. How high you ask?
There's no official threshold but I view it as say in excess of say 7% or 8%. Barley Wines of 10%+ are really the best candidates.
Something like your Vanilla Porter at 5.4% doesn't seem like a good candidate here. However if it's been kept cool and in a dark place like you say then perhaps it will still be drinkable. Although it may also have started to oxidize and taste stale due to the nature of the roasted malts. The balance will also have shifted and the vanilla may not seem as pronounced.
If the product was pasteurized it may negate some of the oxidation, but a brewery like Breckenridge is not likely to do this. It's probably been filtered but very likely not sterile filtered.
All to say that I'd pop one and see what you get. Beer isn't like wine.
Not such a short answer after all I guess!
HTH
:afro:
Cheers,
RR