[bzcomputers]

Having multiple forum.php files within a single installation is not possible.

Each instance of vB would require it's own full installation in order to function in the first place. So, in doing this there would be no conflict and you would just change the skin settings in each individual installation to what you'd like.

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If you are asking if you can offer different skin options for the same vBulletin installation (single license). You would either create or upload a new style and then set this other style(s) as an option in the Style Manager. It would then be available as a user changeable option (typically the option to change styles resides in the footer).