When talking about alcoholic drinks, "dry" is just the opposite of sweet. Vermouth has a slight sweetness to it, so use less vermouth to make the drink "dry".

Personally I like my martinis "dirty" (extra olive juice), with a good amount of vermouth. Sweet and salty with the floral notes of the gin in the background. If I wanted to just taste the gin then I'd put it in a shot glass and drink it straight instead.