Quoting here from a proximate thread (because it concerns old NP):NP 375s keep very well in my experience (that's how I've normally bought it to have on hand). And again, dry French vermouth is handy as an herbed wine in old-fashioned cooking (like, chicken with a wine-cream sauce, pearl onions, mushrooms, serve over rice; or cold chaud-froid sauces). It was often specified in recipes, through about the 1960s.
I've taken to stocking up on old formula Noilly Prat in 375s. I know it won't keep forever ... I've developed quite an affinity for a splash of it on the rocks with a twist, as well as vermouth heavy cocktails like the Bamboo.
Being old enough to remember the 1960s somewhat, I remember Vermouth was commonly ordered in US restaurants as an apéritif, then and somewhet later, compared to today. General US public also drank cocktails much more than wine then, which gradually shifted. Like cocktails, Vermouth seems to've been reborn.
I'm sure it'll be fine in the medium term, but I don't think I'll really want to explore what happens to Noilly Prat with ten years of bottle age.
i bought a couple bottles of cinzano reserva dry last year and though there is no references to them on the web, i think i've dated them to the mid nineties from a strange PR claiming an attempt to re-energize the category with the product in the canadian market.
i'd drank all but one. mainly unmixed. at ten plus years the wine is frail but not dead. there isn't much sugar to give it longevity which is probably more important than alcohol but who knows...
but similarly i've been drinking Pineau Brillet, pineau des charentes and it has a reputation for aging a bit, but floc de gascogne with a really similar alcohol, acid, sugar structure needs to be drank within a year. the floc even has a tiny born on date. i just bought one with out looking at the date and at three years old it tastes really frail if not dead. there is absolutely no fruit in the aroma as opposed to the killer apple-y Brillet. makes me wonder how my bottles of 2004 jean de lillet is doing...