Create site programmatically for WPMU

My customer has a site, which has WP blog + vBulletin forum. Now he wants to allow vb users to have their own blog, by modifying current WP blog to WPMU network.

So I want to create a button, which could be clicked by vb user and new site will be created in network for him. How can I create new site programmatically for WPMU network?

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