What does Vigilance do for creatures? Based on the TCG, I imagine something like allowing creatures to both attack and block, but it seems that Defender already does that.
Vigilance in MTG Puzzle Quest is one of the abilities that has the least to do with the corresponding MtG ability.
Vigilance in MTG Puzzle Quest is similar to defender. It means that if you have no creatures with "real" defender, a creature with vigilance will block your opponent's attacks instead.
You can basically treat it as equivalent to defender.