Is it true that dong cannot be taken out of the country?

Not true,

You can sum up to no more than 15.0 million VND

We have more than two million dong left, we transported it without any issues

Import and export of cash and checks.

According to the customs regulations of Vietnam (specifically Regulation 15/2011/TT-NHNN dated August 15, 2011), an amount in currency equal to or greater than 5,000 USD and an amount of 15 million Vietnamese Dong must be declared. A person possessing cash equal to or exceeding the specified amount is required to fill out a declaration.

Export also occurs with mandatory declaration of funds starting from 5,000 USD or 15 million Vietnamese Dong. It is only possible to export more currency than imported (according to the declaration upon entry) with a certificate (permission from the National Bank). The entry declaration is valid for 12 months from the date of customs control.

ps. If nothing has changed

I don't know what cannot be exported where. More precisely, I do know. But usually, if you don't have a suitcase full of money, no one counts it. For example, I took out $10,000 in cash from Moscow when I flew to Vietnam for the first time. The limit without any declarations, etc. is $10,000. They were just in my wallet. No one said a word. No one tried to count. When I returned, I had about $3,000 left in cash. Similarly, no one tried to count or check. They might selectively fish out and count if a stack of money raises suspicions. Or it should be visibly clear during the inspection that you have five times more than allowed. I have no other explanation for how they monitor that you don't export more than allowed. If you take a couple of bills as a keepsake for your collection, no one will say a word :)