a fair amount of vermouth in a gin martini.  Though I love a vodka martini (James Bond drank vodka "martinis", so there!) it doesn't need the vermouth.
With all due respect to 007, there's no such thing as a "vodka martini".  A 
martini is gin with an infusion of vermouth (classically introduced by pouring vermouth over the ice, pouring it away, then shaking the gin over the same ice and serving "straight up").  That's it.  Anything else is a cocktail, but 
not a martini.
Also, "shaken or stirred" is not an option.  A true martini is shaken over ice.  End of story.
I'm not being a snob here:  I drink a vodka "martini" as I can't stand a "gin" martini.  Doesn't change the fact that my "martini" is simply not a martini.  It's like ordering a Waldorf salad without apples, and cashews instead of walnuts - it's simply not a Waldorf!