...or more generally speaking in the Western Hemisphere south of the U.S. border.
Instead of Vietnam escalating or even occurring at all, create a similar Communist wave of revolution to take place in Central America, the Caribbean, Mexico, and/or South America that America sends troops to combat. POD of your choosing, but perhaps the situation is irritated or initiated by Guevara, an exiled Castro, or an analog.
Bonus points if the conflict truly mirrors Vietnam and extra bonus points if the conflict even if only very briefly reaches American soil.