How can I find a voluteer group to help an 80 year old disabled Vietnam Veteran?

To help in what manner, may I ask?

Call your local Veterans Services office. They help vets line up care and benefits with the VA. Mine is located in our county courthouse.

What kind of help do you need, and how often?

Have you already checked with the VA to see what they might have to offer?
Otherwise depending on what this veteran needs help with, you can check with their church, Senior Services, or Area Agency on Aging.