Cirio H. Santiago's film NAM ANGELS from 1989:
Jack Starrett's film THE LOSERS (re-released on VHS as NAM ANGELS) from 1968:
Both films were shot in the Philippines and both films are about a group of Hell's Angels bikers who take their bikes to the Vietnam War to kick ass (that's my understanding anyway but so far I haven't actually watched either of the films, LOL). The Santiago film is available on DVD from Roger Corman's eBay store and THE LOSERS is out on DVD from Dark Sky and is available from Amazon.
Thanks to Simon for the tip on the Santiago film (but I already knew about it, lol).