If you've got shares, you can either hold them or find a private buyer. Often a private company will have first-right-of-refusal but would have to match a private buyer's price.

Since you're using the words exercise and vested, maybe you have vested options in which case the policy (by law depending on jurisdiction) should tell you how long you have to buy your shares, how the price is determined and when the company has to respond with the strike price. IANAL but giving you 90 days to exercise your options and waiting 91 days to tell you how seems illegal in several ways.