Login to AdminCP.
In the left menu, go to Languages & Phrases. From there, go to Search in Phrases.
Select the Phrase Text and Phrase Variable Name radio button under the Search in... area. Search for text: x_powered_by_vbulletin
Now go and search!
You should have one result, click edit and enter the following into the English (US) Translation text area: {1}
Click on save and you are done!