This is one of the most common questions.

How to Remove the "- Powered by vBulletin" in page title tags

Go to your ADMINCP> Language & Phrases> Search in Phrases

In your Phrases Search for: x_powered_by_vbulletin (be sure to choose to search by Phrase Text and Phrase Variable Name)

Edit the phrase. Simply change it from {1} - Powered by vBulletin

to {1}

So your just deleting - Powered by vBulletin