What should I do to keep Gold Vermeil looking as good as new?

Gold vermeil is delicate in certain ways so take care to use very gentle cleaning with it. Don’t use a sonic cleaner as it can cause the plating to fall away from the base layer.

Here’s a good general article on history and care-
http://www.ehow.com/about_4710262_what-gold-vermeil.html

Agree with @gemiwing . Stay away from the ultrasonic leaners. A gentle, nonabrasive liquid (no heavy solvents) and a soft cloth is what you need.