Do I need to get a nexus letter for my Jet Engine Mechanic father who died of lung cancer & COPD?

I keep getting conflicting information from multiple VSO officers and information online.

They weren't even in the know about my situation so I am disheartened.

He served domestically during the Vietnam era for a period of 1 year 1 month and 20 days.

I was told they have a duty to assist me but I understand VA doctor favor negative classification.

What should I do??