I don't even add vermouth, I just show the bottle to the martini and leave it at that.
For you likker-heads, the reason bombay sapphire is so light and smooth is because unlike Tanqueray and Beefeater 'london' gins, the herbs are not steeped in the mash and then distilled, rather, the pure mash is distilled without herbs, and the vapors from the distillation are allowed to pass through a basket containing the herbs before moving to the condensor. Its a special sort of still arrangement called a 'gin head' and it produces the light sublime flavor of bombay sapphire, and is preferred by many over the 'stouter' flavor of a steeped gin.
peace,
Sam