http://homepage.ntlworld.com/neil.shuck/bgv.html
The attached site is a beginners guide to gaming Vietnam in 28mm and gives some good comparative information on manufacturers, vehicles etc.
In addition to the manufacturers Dave has mentioned there is also
The Assault Group (TAG) - some nice figures
Parkfield Miniatures - new range, haven't seen them in the flesh
Chiltern - big and chunky, not really in scale with anything else
Force of Arms - nice range but not my personal favourite
Recon 28 - are a new range of 28mm Vietnam figures by Ebob. I don't know if they are yet available but they look good.
http://www.recon28.com/
Old Glory have some nice boats