In Admin CP, go to Language & Phrases and do a search for "powered_by_vbulletin". In the open box just enter what you want. As Allan pointed out, that is the minimal wording you can use legally.
To remove the version in the Meta tag go to Common Templates --> headinclude. Find:
Code:
<meta name="generator" content="vBulletin $vboptions[templateversion]" />
And replace it with:
Code:
<meta name="generator" content="vBulletin" />