Prezi should provide more Unicode Fonts that support Vietnamese and other languages with symbol in characters. At present, only font Open Sans Bold can do that.
Thank you for your feed-back! Yes, it’s not available yet, but we are always working on new features, check time to time this site: prezi.com/learn/new-features - and you will be noticed from the first hand about upcoming new features. I also added your request to the wish-list!
Thank you again!