Now the remaining 75% of those options might vest monthly over the next three years. "Vesting" does not impart value. It's not indication of whether you're going to make any money at all. Vesting is an instrument used to make sure you stay on with the company for an appropriate amount of time before you're allowed to own part of the company at that discounted rate.
Point: no, you cannot exercise options at grant time, you can only exercise once vested.