For the more advanced learners, try reading the most significant work of Vietnamese literature, Truyện Kiều (the Tale of Kieu) by Nguyễn Du, entirely in Vietnamese!
On a side note, check out these cool alternative những bảng chữ cái tiếng Việt:
Hm. I would never recommend this one for learners of Vietnamese. It's an advanced read for natives! That said, it's utterly beautiful and an entirely worthy endeavour for those who want to tackle it. The writing is superlative and beautifully lyrical, and the language alone makes reading it desirable. That said, to fully understand the story, you really have to understand the nuances of a collectivist society and the idea of love, both romantic and filial, within that society. I suspect it'll be quite a different reading experience for those who grew up in individualistic societies, but having no first-hand experience, I couldn't be sure of whether it'll be a positive or negative thing.
You said "advanced learners." I said that I wouldn't recommend this for learners, which includes advanced or otherwise. There's more to Truyện Kiều than just the story. If it were just a prettily written story, than yeah, advanced learners would do fine with a little persistence and stubbornness, but that's not all there is to it.