Are you trying to purchase Lightroom 6 through a Creative Cloud subscription? If so, purchase is currently not possible in Vietnam directly through Adobe. Please check the second link John T Smith provides to find a reseller. More information about global availability is located here: Countries and Languages FAQ: Is Creative Cloud available in my country?
If you are trying to purchase a standalone version of Lightroom 6 rather than a subscription, you may be able to access it through Adobe's Southeast Asia store: Adobe Store - Adobe Photoshop Lightroom 6. Alternatively, try a reseller: Other ways to buy | Resellers | Adobe Southeast Asia.
Please remember that, for Adobe purchases, the country associated with your Adobe ID, your method of payment and your purchase location must all be the same. In other words, if you are living in Vietnam but have an Adobe ID and credit card from a different country, you should purchase the program from the Adobe ID/credit card country of origin instead of from your current country.
In case this doesn't answer your question, I am moving your inquiry to the Photoshop Lightroom forum, where the experts are best equipped to help.