It's hard for me to think of any other cocktail ingredient that seems to be in everything, and yet, so few people seem to give a shit about. With that said, it's about time we gave Vermouth a careful look.
I'll try to distill what I know about Vermouth into a few basic recommendations in order for it to be of some benefit. Admittedly, I only barely know my ass from a hole in the ground when it comes to fortified wines, but I'd say if you know the same things I do you're going to be ahead of the pack and on your way to making some next-level drinks. Let's get into why.
So first: what the hell is this stuff?
Vermouth is what we call a fortified wine, which are wines into which additional alcohol and ingredients have been added. Consider that a glass of wine is usually somewhere around 12% ABV, whereas a lot of your vermouths are going to be somewhere between 16 and 18%. Vermouths are also aromatized, which is to say producers can toss in any number of botanicals in there, like lavender, mint, or wormwood. Wormwood, incidentally, is so common that "vermouth" is actually a derivative of the old German word "wermud."
As with a lot of spirits, alcohol often serves as a flavor vector, so the pungent, aromatic nature of the base wines and botanicals are enhanced by the higher gravity of the liquid. Europeans have been known to drink this stuff straight (and actually, a good red vermouth on the rocks is an interesting experience), but typically here in the States we just add it into things, and we tend to do so fairly uncritically.
Incidentally, most of the stuff we add vermouth into tend to be literal staples of the cocktail world. And when the majority of those staples are basically three-ingredient drinks (if you count a few dashes of bitters), the vermouth ends up doing a surprising amount of heavy lifting. But how do we make sure this amount of heavy lifting is good, like a neighbor helping you move a bookshelf, as opposed to bad, like being moments away from a horrible forklift accident? Well...
Rule #1 - Buy good Vermouth
Some people swear by age-old stalwarts like Gallo or Martini and Rossi. It might be that in your neck of the woods, these are the only brands you can get your hands on. If so, they'll do well enough. However, remember that vermouth is basically wine. Once you taste a wine that actually tastes like it's supposed to, it's hard to go back to the litany of jammy, samey, also-ran alternatives that are a bit cheaper. It may not be a fair comparison to me, but Gallo and Martini and Rossi are basically the vermouth equivalents of "Two Buck Chuck." They're cheap, and you'd drink them for free if you were at a dinner party or art gallery opening your significant other dragged you to, but they're not noteworthy.
I'm not going to claim expert status on vermouth, as I've only really tasted about six or seven different brands, but I will share my two favorites. Dolin and Noilly Prat both make vermouths that are a big step up in terms of softness, flavor, and aromatics. The Noilly Prat white (dry) vermouth makes for a killer martini with any juniper-forward gin, and the Dolin red (sweet) vermouth is killer in terms of the sweet earthiness it lends to staples like Manhattans. Try a side-to-side comparison of your usual vermouth brand and the fancier stuff and see if you're not convinced of the difference.
Rule #2 - Use enough of it
Let's get away from that silly Winston Churchill "point the vermouth in the direction of Italy" shit when it comes to making martinis. If you don't use any vermouth, you're drinking gin. That's fine. I drink a lot of gin—mostly straight—but a martini without vermouth is like a watch without an hour hand. Anyone with any familiarity with the invention will know it's fucking missing something.
Consequently, most of my house recipes tend to run fairly "wet," especially for Manhattans where I use a 2:1 ratio of whiskey to vermouth with a very generous supply of bitters. In my mind, if you're going to make a cocktail, make a cocktail. By this I mean that the ingredients should do something to transform the base spirit, complimenting its weaker qualities rather than fighting with its strengths or subtleties. You bought the good stuff: allow yourself to taste it! You might be surprised.
Rule #3 - Pitch it when you need to.
This might be the most important tip I have to share. Consider that most people treat their vermouth like they treat their spirits, which is to say that they figure it can sit on a shelf for decades with the knowledge it will always be ready for immediate cocktail use when duty calls. Not so.
See, I did not know this when I was a younger man.
Which is why for years I hated martinis and manhattans. As we said before, vermouth is a fortified wine. And like wine, it can and will turn to vinegar when exposed to air and warmth. It does it more slowly than a bottle of wine, yes, but it will happen. And then—when that day comes that you decide to make a cocktail—no matter how laboriously you shake your martini like James Bond, it will taste like hammered asshole.
But here's the best explanation of why I think people don't immediately blurt out, "My god, this is disgusting!" when they have a cocktail made with a vermouth that turned (perhaps years ago): the aromatic and sometimes savory nature of vermouth—along with what's usually a shitload of booze—mutes the effects of the vinegar from coming through like a shriek in a library. Unless you normally drink martinis or Manhattans and know what a good one is supposed to taste like, you'll just mix one up with whatever ancient vermouth that's been moldering under the sink and conclude, "Yuck. I don't think this drink is for me."
In terms of keeping my vermouth fresh, I tend to buy the smaller (375mL) sizes of vermouth. I then keep them in the fridge and vacuum-seal them with these guys in order to max out their shelf lives. If a bottle has been in the fridge for more than four months, I toss it.
That's not an absolute rule of thumb, but so far it's worked generally well enough for me to be able to experiment with the staples and other cocktails that call for either red or white vermouth. The 375mL size is about 12 ounces, which even if you make your drinks pretty "wet" will generate about 12 cocktails. That's plenty for entertaining. Unless you drink a martini every day and that's been your "thing" for years, there's really never a need for the big bottle.
Rule #4: Have fun.
Again, don't fear the vermouth. Probably most of the people telling you to make a martini extra extra dry are people who tried one awfully shitty martini years back that was made with some Cinzano first opened during the Clinton presidency. Or, they're people who have really only had dry martinis because they're fashionable and lack an alternative of anything they can compare it with.