Just got this from the VUDU forums from a question I had asked:
Hey newbie here and just joined this forum! I have used Vudu a couple of times and even did one DVD disc to digital upgrade to HD. I just noticed that when I go to that particular movie on my Xbox it now says watch in HDX not HD. My 360 did an update as I started the app this evening. Does the 360 now do HDX? My strength bar even had HDX. And while I'm on the subject. If I happen to make a HDX purchase will it still give me the HD or SD version depending on the device I'm using? Thanks.
You can now watch HDX on your Xbox 360. If you purchase HDX, you will be able to watch that version account wide. Keep in mind that PC/Mac is currently restricted to SD only.
VUDU Customer Care