In order to unlock Videl, you need to be in Adult Gohan's Dragon Universe. Before you fight Debura, search the fields east of Goku's house. After this first event with Videl, search the islands in the lower right part of the map. Find the spot where you have a second scene with Videl (it's about her being/needing training). After this scene, Videl will become unlocked. Good Luck!
Death is but a door, Time is just a window,...I'll be Back!!!!!