On the old Pardot landing page, click edit. On the first page, where you name the page and declare the vanity URL, you want to remove that vanity URL (save it for the next step).

Click Save. (Now you have freed up that URL for use elsewhere.)

Go to a Custom Redirect. Create a new custom redirect (name, campaign, etc.). Take the vanity URL from your old page, and put it in the Vanity URL box here. Put the URL of the NEW page in the "destination URL" box.

Save.

Now you have created a redirect from one Pardot URL to another.