How do I make Vietnamese Yogurt more tart? How do I make Vietnamese Yogurt more tart?

I've been using several online recipes to make Vietnamese Yogurt. The ones I buy from the store are much more tart then my own results. 

My results:


Danactiv vanilla flavor - Way too sweet, but texture was alright
365 Organic Plain - Not too sweet, but not tart at all, texture was very thick


I have not been able to go to a store that sells Vietnamese yogurt due to the lockdown, so I didn't try that, yet. Hoping for an answer that allows me to make tart yogurt American grocery store ingredients.

Here's an example: https://www.youtube.com/watch?v=AgNtOOa8dg8

For incubation we used a warm water bath and left at room temperature (74F)
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.