I've seen various versions but not sure what would be an acceptable polite phrase. In Mexico a bartender told me it's "ME DAS un mojito, por favor", in Barcelona a waitress told me I can just say "QUIERO un vino, por favor". Both seem a little too demanding and not polite translated to English so I'm not sure I should be using them.
There are, of course, many ways to ask for a drink, and most of them will have the same basic result: You receiving a drink.
As you imply, some are more polite than others.
If the bartender has already asked you "¿Qué te gustaría tomar?" (or similar), you can politely respond pretty much as you suggested:
Quiero un vino, por favor.
If you have to approach the bartender/server, then you may want to be a bit more polite.
Disculpa / Perdona. ¿Me traes un vino, por favor?
Or as my wife (who lived in Madrid for 10 years) says would be far more common:
Perdona. ¿Me traes un vinito?
The diminuitive on 'vino' adds some 'kid gloves' to the statement, making it a bit more polite.
She also points out that in Madrid it's uncommon to use usted except when addressing your superior (in age or position), so wait staff would generally not be addressed with usted.