Decline grad school for fellowship?
A major fellowship may be enough to get a grad school to agree to a longer deferment. However, even if they're not willing to extend you a two-year fellowship, when you reapply, you will have that major program under your belt, and will be a stronger candidate than many in your field applying straight out of undergrad. 

So I wouldn't worry too much about this in terms of your long-term grad school prospects.