My dad cosigned for my brother about 10 years ago. The vehicle was
Repossessed and I think the debt is upwards of 20k now. Last year my
Brother filed bankruptcy.
Recently my parents have been getting calls from some collection
Agency. I heard since my mother didn't sigh they can't go after my
Dad's assets since they're in both their names. However if she were
To die they will be ready to do that. Is this true?

Instead of ignoring the calls, should they deal with the problem?
I've heard of Debt Negotiation, how does this work?
Any advice out there?

Was the loan retired under your brother's bankruptcy? That could have released your father's obligation. Otherwise dad IS obligated. When you co-sign you assume responsibility for the debt.