If you mean at different forums from an end user experience, most only offer one or the other. Some (like WT) allow the user to choose in their User Options.
If you are the administrator of a vBulletin site, then you'll want to use the vBulletin support forums or contact form. Unless you are using a pirated version, and in that case you should submit your URL to this form to get your site taken down sooner rather then later.
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