You'll first meet Vesna inside the country inn in the Outskirts of Vizima. She'll manage to resist your charms if you flirt with her there, but at some point later, during the evening hours, you'll run into her on the road near the inn.

When you meet her on the road, Vesna will be surrounded by bandits. If you rescue her from this predicament and then escort her home, she'll suggest that you meet up with her at the Old Mill, and that if you bring the wine then she'll bring the food. This rendezvous naturally will lead to you having sex with her.