Over 10 years ago now but...Cu Chi (for the tunnels) definitely worth a look, and the Mekong Delta was good. Wasnt as captivated by Ho Chi Minh (Saigon). Hoi An is nice, nothing much to detain you in Hue. Really liked Hanoi, great food. Halong Bay is worth a look, and there's a boat trip to Perfume Pagoda that's great. All v touristy but "authentic experiences" can be found in any semi-d in Ireland, you go abroad for the good bits.
You can bus it up Highway 1, wasnt as bad as predicted from what I recall.