Perhaps the question should be, how good does your programming have to be before you become capable of love?
The Vision's heuristic software allows him to learn, grow and evolve. While he was initially created to infiltrate the Avengers, it was assumed he would have to have sufficient ability to look and learn in order to interact effectively with the Avengers.
If you look at his history, he starts cold and unfeeling but slowly develops as a person, both in terms of his experiences and his willingness to sacrifice himself to save the other Avengers. Self-sacrifice is a sign of the power of choice indicating his Heuristic programming is not only replicating human behavior based on the engrams of Simon Williams, but evolving as a person to become a complete human being.
His ability to show emotion, is one that has taken time to develop from his experiences and a sign of his programming and his emotional database from the mental engrams of Simon Williams. The Vision has evolved to become a new and complete being with his own emotional spectrum.