A Dong does not have decimals, hence you should not add decimals in customizing. (keep in mind such change cannot be revoked).
I think it is just a matter of entry sequence in your screen.
SAP can only know about the currency of a plant after you entered the plant and have pressed enter. as only hitting enter is submitting values and triggers the PAI event which kicks the program to go further.
In general has a currency entry field 2 decimals, only after entering a plant (and pressing enter) SAP can change this.
So I would suggest you enter all information except the valuation, then you press enter, then enter the valuation.
Let me know if the different sequence works for you.