Some good advice here; couple qs: did you get shares for your personal capital contribution?

Simplest thing to do if you think it will work and don't need money is just sit on your vested shares, perhaps ask for a little more as an exit package, be supportive, offer to join the board as an advisor, and move on.

If you'd like a clean break and some cash, you could call your last round investors and offer them a discount to the last price for your vested shares. Depending on the state of the company and what's next, they may be interested -- there's an implication in these conversations that you'll find SOMEONE to buy eventually, which they may or may not like. I'd guess unless things are gangbusters, you might look at 30-50% discount from the last valuation, knowing nothing about the common, the company or the pref stack.

Another possibility - you could offer to sell your shares at a discount to your cofounder, leaving him in a good spot - he could ask around for a loan / investment (in which case the company would be the buyer, and he'd benefit pro-rata).

Personal advice - keep some, sell some, stay available to help.

I agree with Joel Spolsky that nobody should get shares for personal capital contributions (at least, not unless they're investing in a round). If you want to be fastidious, set up IOUs.