It has a list of Khe Sanh veterans with email addresses, including their units as well as a message board. There is also a Khe Sanh Veterans Association which holds reunions, prints a hardcopy newsletter, and has a directory of its members. I would strongly recommend your contacting the newsletter editor (who is not on email that I know of):
Stubbe is the co-author with John Prados of "Valley of Decision," one of the best combination oral and scholarly histories of the Vietnam War about the siege at Khe Sanh and the hill fights. He is most likely to be your best resource in finding Marines who served.