I don't know how reliable source, but I've pasted what they have to say about it below (https://www.employmentlawfirms.com/resources/will-my-severance-prevent-me-collecting-unemployment.htm").
"In this regard, you are lucky to have worked in California. Under California law, severance pay is not considered wages for unemployment purposes. Instead, it is considered a payment in recognition of your past service. Even if it is paid out in installments, as yours will be, it doesn't count against your unemployment."
"To count as severance pay under California law, your severance must be paid out pursuant to an employer policy or plan. And, it must be available to a class or group of employees. Based on the facts you've given, it sounds like your severance could meet these criteria. (To learn more about these requirements, see Severance Pay, Dismissal, or Separation Pay, at the website of California's Employment Development Department.)"
I am in the same boat you are - based on my reading of this I will probably declare both vacation pay & severance pay......and hopefully this will not impact my eligibility for unemployment benefits.