2200 years ago, the "Viet" nationality inhabited the coast of southeast Asia from near Shanghai south to the Red River plain - what we would now call Northern North Vietnam. Just before the study period began, the Chinese under the Han Dynasty took over most of the region. All but the southern Viets (Nam-Viet in Vietnamese) were assimilated into Chinese culture. The Red River plain, although ruled by China, retained it's ethnic identity while absorbing Chinese culture. Thus, if your family was in Northernmost Vietnam in CE 500, their ancestors could also have been in China in CE 1.
A significant side effect of the expansion and cultural homogenization of China was the immigration of members of aboriginal non-chinese nationalities from southern China into southeast Asia. These include the ancestors of the Hmong, who are derived from the Miao nationality of southern China. If your ancestors were Hmong in Vietnam or Laos in CE 1800, their ancestors wouldhave lived both in Southeast Asia and in southern China in earlier periods.
What we now call southern and central Vietnam was not ethnically Viet. Culturally, its inhabitants were strongly influenced by India rather than China.
Tenth Century: The Viets of the Red River succeeded in giving their Chinese overlords the boot, establishing the kingdom of Nam-Viet (The name Vietnam was coined in the 18th century). At this point, ethnic Vietnamese began spreading to the south. This push, the "Nam tien" (March to the South) was the work of generations, analogous to the Medieval German push to the East.
Adding fuel to the March to the South was the fact that the Vietnamese were frequently occupied repelling Chinese attempts to absorb them. When each war ended, veterans would be rewarded with lands in the South. By the eighteenth century, Viet settlers gained the upper hand in the Mekong Delta, the region's bread basket. Thus If your ancestors lived in central Vietnam in CE 1490, they would have also had ancestors in northern Vietnam in CE 1000. If your ancestors lived in southern Vietnam in CE 1800, they would have also had ancestors in northern and central Vietnam in CE 1490.
Sixteenth Century: European powers (Spain, Portugal, France) began to take an interest in Vietnam.
Nineteenth Century: The winners were the French, who established control of southern Vietnam by the mid nineteenth century, then extended to the north, meeting determined resistance at all points. By the 1880s, they were able to settle French administrators and distribute plantations to French settlers. Unlike in the Philippines, where Spanish authorities coopted a native elite to administer the country for them, Vietnamese natives had little opportunity to participate in the French colonial economy. Still, there were certainly opportunities for the French to spread their genes.
The subsequent history of Vietnam is well known. If you are of Vietnamese ancestry with French, Japanese, or American admixture, your family folklore will tell you.