At 80%, you can apply for IU individual unemployability - assuming that you are unable to be gainfully employed because of your service connected disabilities. Your best be is to go to your POA (power of attorney) to file the claim for you. By POA, I mean a service organizations (such as DAV, VFW, American Legion, etc.) they help Veterans obtain their Veterans benefits. They can explain IU and help you file your claim. And if you have already applied for it and were denied (if within the one year period since the decision, you can file a NOD - notice of disagreement). It is worth your time to do this!
With Vietnam era service, there are many “presumptive” conditions that the VA has identified - again your POA can explain and assist. PLEASE look into this!