With vBulletin you only use one Account Name, so it defaults to your UBB User Name, which was different from your Display Name.
To Change this,
Forum Actions at the Top > Edit Profile
Then you can change your User Name to something other than your Old UBB User Name.
To Change this,
Forum Actions at the Top > Edit Profile
Then you can change your User Name to something other than your Old UBB User Name.