how do i upgrade so i can use vantage theme on more than one site?

Hi Oscar

Thanks for your interest. If you upgrade for $35 (as you’ve already paid $15) and then drop me a mail on [email protected], I’ll mark your second purchase as multi-site. Alternatively, you could upgrade each additional domain.

Hope that helps.

(Ref: https://siteorigin.com/thread/how-to-purchase-the-premium-version-of-a-siteorigin-theme/)