Stateside, Vinamilk yogurt is impossible to find; I've been on a hunt for the flavor of Vietnamese yogurt for months by the time I found an excellent replacement at Banh Cuon Tan Dinh (2850 SE 82nd Ave. Ste. 11, Portland). During my search for the perfect yogurt, someone was kind enough to post the recipe their mother uses in the comments of my travel blog. I tried it once, using plain yogurt from the local Safeway and though it wasn't the right consistency, it was a close approximation. After finding the real thing at the restaurant, I made it again and it was a delightful discovery of the flavor (and consistency) that I had so missed.
If you have a Vietnamese grocery or restaurant nearby, look for real Vietnamese yogurt to use as a base, but if it's not available, feel free to use plain yogurt from your local grocery.
Boil some water in a tea kettle while you pour a 14-ounce can of condensed milk into a heat-safe bowl (or a large saucepan). Using the empty can as a measuring cup, add one can of boiling water and stir with a wooden spoon until condensed milk has completely mixed with the water.
Stir in 8-10 oz. of yogurt and continue to stir, mixing until there are no small chunks of yogurt left. Stir in 2 cans of milk (using the condensed milk can as the measure) until thoroughly mixed. Now, you've got your yogurt mix.
Using the small bottles that you've collected, pour the yogurt into each cup and place them in a deep pan or tall soup pot as each is filled. When all the bottles have been filled, pour boiling water around the bottles until it reaches the necks of the bottles. Be careful to not get any water in the yogurt.
Now cover the pan with a towel or two, making sure that it doesn't touch the tops of the yogurt (one reason a tall soup pot is a good choice). Let it stand for four hours, doing its culturing bit, then remove from the pan, screw on the lids and stick them in the fridge. Refrigerate overnight or for at least four hours before eating. It'll develop a sort of dry crust, that's normal.
Make sure to save one bottle aside to use for the next batch, if you can resist eating it all!