Below is a newspaper report of January this year and it says because of trade war between the USA and China, the Vietnamese Dong will depreciate this year in terms of dollar which may be already happening. So, in this case Vietnamese export prices will be cheaper almost at the same rate as its depreciation.VND is linked to chinese currency the Yuan. If Yuan goes up against USD then VND goes up. Same in other direction.
In the case of BD, the Govt is not allowing a Taka depreciation. I do not know the real reasons but one could be that the GoB likes its GDP shown superficially higher than the actual. Another reason could be the rise of prices of imported items in Taka which will cause a decreased import that will again cause the import duties to decrease forcing the govt to borrow more money from banks and issue more bonds in the market in order to cover Budget deficits.
Excerpt:
"Moreover, as US President Donald Trump wants to use the trade war to gain an advantage over his opponent in his re-election campaign, the war cannot end in 2020, Hieu said, adding this would continue to have a strong impact on US dollar and Chinese yuan in 2020, putting pressure on the dong.
“The impacts will cause the dong to depreciate by between 2 and 3 percent in 2020,” Hieu forecast".