Abre los Ojos (in english, Open Your Eyes) is the Spanish film on which Vanilla Sky was based. The plot of Vanilla Sky follows that of Abre los Ojos fairly closely, but varies somewhat in thematic emphasis. In Ojos, for example, Life Extension is seen as a scientific alternative to the Catholic claims about the afterlife, whereas in the American version the issues raised have more to do with authenticity, personal fulfillment, and cool special effects.
The difference is best captured in a particular scene found in both films, in which the main character goes on a drive with an ex-girlfriend. As they are driving along the road, the ex turns to him and asks “do you believe in God?” In the Spanish version this comes across as a moment of deep reflection about the meaninglessness of a post-Christian world, whereas in the American version, the question is seen basically as a signal that the girl is stark raving bonkers.
Penélope Cruz appears in both films playing the same role, but the personality of the character in the two films is quite different. In Vanilla Sky Cruz comes across as a kind, nurturing, girl-next door type, whereas in Ojos she seems more stuck up. Whether this is supposed to be representative of some deep difference between Spanish and American culture, or whether it was even intentional I don’t know.
Ultimately I thought Ojos did a better job thematically, though it couldn’t really compete with the production values of Sky. If you have already seen Vanilla Sky, I would recommend Abre los Ojos. If you haven’t, I would recommend both pictures, though I don’t think it much matters in which order you view them.