Is it correct to say "Voudrez-vous une tasse de thé?"

Yes, it is. Although more polite is "voudriez-vous une tasse de thé. " -- Chris (not a native speaker)

Thank you!!

Although grammatically correct, Vous voudrez une tasse de thé ? will relate to an event in the future, meaning something like, Will you be wanting a cup a tea?

For, Do you want a cup of tea? you will say, Vous voulez une tasse de thé ? (or voulez-vous une...) more than the polite form Voudriez-vous une tasse de thé ? which sounds stilted. But in shops or in the case of any dealings with strangers you will use "je voudrais", not "je veux" which sounds rude in French!