I am tempted to write about the specific moments in the movie and analyze how simply creative woody Allen is. But that would knock out some surprise if you plan to watch it. So...
I think Woody Allen explores brilliantly how the society evolves in relationships and as in any of his other movies, it's so much fun to watch him play through his characters with a light sense of mockery humor. Right from the way the movie starts, the way approaches the girls... their intriguing nature of response... the unexpected way things turn out between the 3 of them... and then the introduction of ex-girl friend... are very unexpected and unconventional, yet shown in a temptingly possible way.
Purely about relationships, Vicky Cristina Barcelona is very romantic and adventurous.
Its a knock at you to take a look at yourself where you are holding back from exploring the unexplored.
The experience of wonder always comes from the place of the unexpected and the uncertain, beyond your reasons.