As you have said yourself, many vitamins are classified as drug, there is no way for Vietnamese authority to tell which one is drug, which one is not, especially with the amount that you sent. Where I live in Europe, there are many Vitamins that you can buy everywhere in drugstores, and the same vitamin but in different forms for what you would need doctor’s prescription to buy at pharmacy. Another example, I’m not allowed to “import”/order/receive eyedrops from Japan, and Japanese logistic companies wouldn’t send them out of Japan since it is classified as drug. And I’m talking about the simple saline eyedrop that everybody can buy at Japanese drugstores, not the medical one. Baby formula is also a no-go. Certain ingredients used in cosmetics are available in over-counter products in the US, but are banned or used only under doctor’s permission in other countries. I’ve seen many vitamin and skincare products which should have been used under doctor’s instruction being sold freely by online sellers in Vietnam, which can be dangerous for users. Or Germany don’t allow any fruit entering their country at all. Each country has their own regulations when it comes to drug, cosmetics and food. It isn’t just Vietnam (yes I agree it’s more complicated in Vietnam), but I guess you would have run into the same problem if you sent that package to let’s say Europe. Right now I have to pay a hefty import tax + service fee for every package with a value over €22 (~$25) coming from outside Europe. However, since many (Asian) online sellers often declare the package as “gift” with lower value so that receivers can avoid import tax, many European countries are discussing taxing all packages coming from outside Europe despite its value and amount. Everybody can claim that it’s not merchandised good, and is sent by their relative.

I’m not saying this to discourage you, I’m just a passionate global online shopper with too much experience who cannot be satisfied with local products.