English-speaking authors have been writing about Vietnam for hundreds of years. So there are certain conventions on how it's done. The Chicago Manual of Style recommends that writers follow the spellings given in the various Merriam-Webster dictionaries. As you can see below, these spellings are consistent with those found in other major reference works.
When a publication has Vietnamese writers and a primarily Vietnamese readership, it would easier for everyone involved if the diacritics were left in. Yet every English-language news site in Vietnam drops them out. Why? Because the point of such sites is to model conventional language usage, not to promote Viet-lish. The most widely read of these sites is Voice of Vietnam. The site’s international style and lack of localisms are selling points, even though the readership is overwhelmingly Vietnamese.
|Britannica||Columbia||Encarta||American Heritage||Collins||Oxford||Random House||Merriam-Webster|
|Ho Chi Minh City||Ho Chi Minh City||Ho Chi Minh City||Ho Chi Minh City Formerly Saigon||Ho Chi Minh City||Ho Chi Minh City||Ho Chi Minh City||Ho Chi Minh City|
|Can Tho||Can Tho||Can Tho||Can Tho||Can-Tho||Can Tho|
|Da Nang||Da Nang||Da Nang or Da·nang||Da Nang||Da Nang||Da Nang||Da Nang|
|Bien Hoa||Bien Hoa||Bien Hoa||Bien Hoa||Bien Hoa also Bien-hoa|