I’ve never had that option.
Seems you are right. But they have no intention of adding it:
https://forum.vudu.com/forum/vudu/general-discussion/445418-vudu-s-new-look
Vudu seems to be having major quality and content issues with MA messing with their plans:
https://forum.vudu.com/forum/vudu/general-discussion/445277-did-vudu-just-give-up-on-uhd