One option we've used in these types of situations: You can enter into an agreement with the company whereby the company is given the option to repurchase your shares (or some portion thereof) in connection with a VC funding round. You can mutually agree on a valuation method (specified fraction of funding round price, 409A price, have a valuation done by an independent accountant). This allows the company to minimize the dead weight on the cap table, while allowing you to obtain some compensation for giving up your shares.
I would not recommend just offering to give up vested shares. What was the point of the vesting schedule you had agreed upon? I assume you've been working without a salary for 16 months, right? What compensation do they think you should get for that?
Happy to provide further advice off this thread, if you have further questions.