If you already have a website with your custom domain, you can set up your domain on Office 365 operated by 21Vianet without affecting your website. Be sure to choose the option in the domains setup wizard to note that you already have a website that you're using with your domain. Or, if the option is available in the wizard, choose to manage your own DNS records for your domain. With both of these options, the wizard will walk you through setting up your domain's DNS records correctly at your current domain registrar.
If you don’t already have a website or if you want to create a new one, depending on when you purchased your subscription, you may be able host a SharePoint Online Public Website on your domain (like www.fourthcoffee.com). While you can’t move your current website to Office 365, if your subscription includes a SharePoint Online Public Website, you can create a new website that has a similar look and functionality.
For information using a SharePoint Online Public Website, see Public website help for Office 365 operated by 21Vianet.