Although Vicky was in love with Sean, he never even looked at Vicky twice. Vicky wasted five years obsessing over Sean, talking about him all day long and never looking at anybody else while he was around. Vicky was warned by Martha many times to "get out" of the relationship, since Sean only liked Vicky and nothing more would happen.
At the end of her travels with the Tenth Doctor, Martha Jones used Sean's relationship with Vicky as a comparison to explain how she felt about him. She also loved the Doctor, but he never looked at this. Because of this, Martha explained that she too needed to "get out," and thus made the decision to leave the TARDIS. (TV: Last of the Time Lords)