How do I make Vietnamese Yogurt more tart?
First, the tangness of yogurt depends on the culture, and somewhat on the temperature of incubation. Lactobacillus bulgaricus will give you a tangier yogurt than streptoci or bifidi-based cultures. So buy a lactobacilicus culture (either the pure culture or the yogurt made with it) and use it as your culture. Then make sure to incubate at the proper temperature (which is 43-46 for lactobacillus bulgaricus). If that's not tangy enough for you, leave the incubated yogurt one night at room temperature before putting it in the fridge. 

Second, sweetness and sourness are an unusual pair of tastes in that they inhibit each other. I have not had vietnamese yogurt, but the Internet tells me it uses sugar. So consider reducing the sugar in order to get more of a sour taste when you eat it.