Make the spring onion-infused vodka: In a large jar, combine the vodka and spring onions. Infuse at room temperature for 12 hours.
Strain the mixture and return the liquid to the bottle. Keep refrigerated.
Make the cocktail: In a chilled mixing glass, combine the vodka and vermouth. Stir until chilled, then strain into a chilled martini glass. Garnish with pickled ramps and serve.