You can still redeem Flixster codes on VUDU. For the Green Lantern, this is the link you could've used:
https://my.vudu.com/redeem.html#selectPromo/_/_/198359/_/_/_/_/_/false/
Or you could've used the generic myuv.com/en/us/redeem link but sometimes that one just redirects to Flixster.
Thank you, I managed to grab Adventures of Tree :) I had originally tried for Shadowgate but for some reason, when I verified, the code had already been used.