Robert Griffin III has gone INTERNATIONAL ... and he's HUUUGE in Vietnam ... so says legendary Bill Clinton investigator Ken Starr.

Starr -- who's currently the President and Chancellor of Baylor -- was cruisin' around Capitol Hill yesterday when we asked how important RG3 has been for the University.

Turns out, Griffin has a pretty rabid following in Ho Chi Minh City ... where Starr says the locals love the NFL quarterback so much, they literally chanted his name when they saw a member of Starr's crew sportin' some Baylor gear.