My son was a member of the labor union for about 4 months before he passed away in a non work related automobile accident. The union listed a $10000 death benefit in thier literature my son recieved when he joined. I recieved a $200 check for his death benefit from the union shortly after he died along with a form to sign stating that I accepted the check as payment in full. I called and spoke to a representative who informed me my son was not entitled to the full amount because he was not 'vested'. I never cashed the check because I wanted to research the law regarding what my son was or was not entitled to at a later date when I was less emotional. It has been over a year since the check was issued and the union called and asked me to please cash it so they could close their books.
Vesting in this area basically means that you have fulfilled all the requisites to be entitled to something. When it comes to benefits, it is usually a time requirement. Most probably the company required a certain length of time employed to become vested, meaning to be entitled to the benefit. You could contact the company and find out what where the requirements for becoming vested and check if your son actually did not meet them.
This does not create an attorney/client relationship. This does not constitue legal advice. It is limited to facts of the question. You should consult an attorney before making any decisions based on this answer.