Widow's disability benefits?

My husband (now deceased) was a member of the USMC in Vietnam 1965/66. He was wounded and received a Purple Heart and also disability benefits. He died in 1998 of a mive heart attack. The autopsy showed that he had an enlarged heart, and I was told by the VFW that this condition was most likely caused by Agent Orange used in Vietnam. I recently learned that I may be eligible to receive his benefits. Is this true? How do I learn more? Who should I call to find out if I am eligible? Thank you for any help you may offer.