If they think they have legit grounds to let you go based on “performance” and are still going to give you a five-months-salary severance package, all other issues notwithstanding, I would generally say take it, as in a lot of other non-union situations they might have legally been obliged to give you only two-weeks notice/severance.
On the other hand, you need to check on any company HR policy regarding unused vacation time. There may be something in there that suggests you are entitled to pay in lieu of that time, regardless of the circumstances of your leaving. But, check THAT part out; I could just as well be wrong on this.
In an "At Will" state, an employer can terminate you for no reason whatsoever.......