On mine, you turn the dial to the heater symbol (6 o'clock pos) and leave it, it will take approx 3 mins to heat up, if it doesnt, go back to the dealer.I have electric folding mirrors too. (dial to 12 o'clock pos). if you are pushing the button then you are not doing it corrent (paulm007).
also if you do use the defrost funtion, do insure that you turn them off once mirror is frost free, because if you leave them on, you WILL burn out the element and is expensive to replace.Not sure VW cover that if you do.