Talking to a lawyer will only do one thing...cost you money. For starters, this is not a layoff. It is a voluntary package being offered while notifying individuals that their position is at risk of not being around come January. What can a lawyer do with you making a choice? Nothing.
Now, you can choose not to take it, roll the dice, and see if you survive in December. I think it is extremely unlikely....
But it is a layoff. The only voluntary thing about it is that you get to choose when to go. You can voluntarily go early with a larger payout or you can choose to stay and hope to land a new job, which as you said is extremely unlikely, and get a smaller package.
Different people are interpreting what the "stay in place" option is and what "at risk" means. If you don't find a new job, are you "at risk" or "are you gone"? The definition of risk is a potential, not a certainty. Where does the risk lie? In your role being eliminated or in you not being able to find a new role? If it's the former, then Cisco has a better argument that you voluntarily left. If it's the latter, then there was no voluntary choice in being let go, only a choice in WHEN you leave: Oct 30, Nov 23 or Jan 4.
Is it too much to ask that the ELT be transparent and tell us what the "at risk" means? I'd bet there are a lot of people thinking they can beat the at-risk and waiting thinking that if enough other people take the early package their role might survive.