Since you can't get access to feed due to the pandemic lets see if we can come up with some better food options that you might have access too. First off, by any chance can you find duckweed? I believe it's called bèo there and it should be a native species of plant/moss? It has 20% protein, vitamins and minerals most importantly niacin. Duckweed is very nutrient dense and at this point I think it's a Hail Mary pass worth trying. Also if you have access to peas they are higher in niacin than many other vegetables. If you have access to whole grains like whole oats or wheat they will be better options than rice for the duckling although you may have to soften them with a quick boil or smash them a little. I don't know what groceries you do have access to currently due to the pandemic, but fish meal or fish? Salmon and tuna especially have the highest niacin levels. Scrambled eggs are also highly nutritious and nutrient dense for ducklings. Do you have B3/Niacin supplements in your medicine cabinet or access to them? If it's in capsule form that's fine you would just need to open the capsule up and dissolve it in their water. On to bugs.. can you find outside any mealworms, crickets, waxworms, superworms, and silkworms? Or really any bugs in general? Bugs are the big thing that help maintain ducks in the wild since they don't have access to specialized feed. If you can look up what the diet of wild ducks in Vietnam normally looks like you also might find some ideas.
On a side note I do not know the culture there so I do not even know if contact is possible or what resources you have, but do you have any zoo's, wildlife centers or something down that line that you could possibly phone or email asking for informational help on ducks? You might get lucky and find someone knowledgeable about ducks or even just waterfowl in general who would be happy to give some advice regarding feeding ducks in your region and the best way to get ahold of resources in this covid craziness.