Please paste the code and a link to this so we have the ability to check out the style. However if it is what I think it is, you should just be able to alter the redirect in the form settings. If you are calling the same core-files that Vbulletin uses, you will be altering it board-wide, not just for that page.
If you wanted it to work both ways, you would have to put a IF/ELSE statement in there doing a verify on the URL. But I don't want to get off-topic here.
Please post the code and we will go from there