I had 21 years in when I quit. I think it used to be 2 weeks notice, plus 1 week per year up to a max of 26 weeks. When the severance dropped from 23+ weeks, to basically 8 if you were lucky, it no longer became a factor in the decision to leave. Prior to that, being the 'new guy' somewhere vs having a 5 month safety-net was a major incentive to stick around.
When I left, any unused vacation was lost - which was convenient after I cancelled two days vacation to help them deal with some stuff during my notice, and then they billed me for the 2 days they over-paid me at the end of the month, but wouldn't credit me the 2 days vacation I had left over. States in the Employee Policy that vacation is lost at the time of separation.
Not sure if it works the same with involuntary separation, but in most cases, if you accrued it, they owe it, but check your local policies ;)