Vergil probably didn't know about Dante being his brother until fairly recently. That's probably the reason why he asked Dante to join The Order (they couldn't come up with a better name?). He probably did it out of guilt....and an invitation to 'higher status'. ("Why do you refuse to gain power?"--That's Old Vergil's idea of an invitation--that, and wrecking his office and area surrounding it with the demons and towers)
Off topic: Why do I see Dante dancing like those guys at The Roxbury after watching this?