Looking for severance advice, what should I expect?

My employer has offered a currently undefined severance, I have been put on an improvement plan for goals reasons and my position is not being eliminated. I had a medical issue in the middle of the improvement plan that caused me to be hospitalized and required rehab. I also have a case filed against a verbally abusive boss who has now been demoted. I'm a 20+ year loyal employee with a clean record in a fortune 50 company and I'm over 55. I have been defamed and harassed by the former boss and have claimed hostile work environment with HR who investigated my claim. I also have a lot of vacation that I can't get paid for and I have been encouraged by management not to use it. I believe my personal choice is to ask for the offered severance and resign. Based on my details, what do you believe should be appropriate as far a severance offer? I also have a 1 year non-solicite agreement.