It obviously can't help if the film you've chosen goes downhill after that. But if you're not feeling what you're watching right from the start, all you need to do is stop the film within half an hour, and then call or email customer service within 24 hours. Vudu will issue your account with credit equal to the price of your rental within the day, as well.
While the feature sounds like a great way to find the perfect films to watch for your holiday get-togethers, take note that it does have limitations. You can only do it once per movie, and you're limited to four redos per month. Also, it won't work on movies you've already downloaded on your device, because Vudu has to be able to remove the rental from your account to be able to issue your credit.
In case your feelings of regret came from renting from Vudu when the movie you've chosen is available for cheaper elsewhere, don't worry: The platform has also introduced price matching for Amazon and iTunes. The rental has to be identical in every way, though, including resolution and edition. You also need to contact customer service within 24 hours and while Vudu can confirm its rivals' lower prices, so you can receive the price difference in your account.