Currently I'm running a campaign that has two vanity landing pages that redirect to my main landing page which has a simple form asking for the leads email. However I want to track which vanity URL the lead originally came from (Vanity URL 1 or Vanity URL 2). I thought the best way would to be add a hidden field, but how can I set it up so it picks the URL it originated from?
If your "vanity URL" is a Marketo-managed redirect, that's not going to provide you with information about the originally-visited URL.
You would have to have an interstitial page that does a JS (i.e. not HTTP) change of location. Then you could check the Referrer, which would be one of the vanity URLs.