The Mishnah on 79b: R. Akiva says that a Saris Adam can do Chalitzah, and I think also Yibum, and I believe that's how we Poskin. Does that mean a man with a Vasectomy can do Yibum? But if we know with certainty that there is no chance that beah can 'raise up his brothers name', why is Yibum allowed - why does this mater the issur of Aishes Ach? Do the Poskim deal with this question, and I'm just curious what the Halachah Le Maisah is (not that it is relevant to anyone I know).
Akiva Lane, Ramat Beit Shemesh
The Poskim do not state that he may do Yibum a priori; they say only that if he succeeded in having intimate relations it is considered a Kinyan and she is a Yevamah. The concept of Yibum would be an obligation even for an older woman or one who is sterile, even though we know she cannot have children, since there was a time that she could, and this is also the case with a person who became sterile during the course of life.
This is because the Mitzvah of Yibum is the act of Bi'ah, irrespective of whether there will be a child born. The meaning of the verse is not that there will be a child born to commemorate the brother (there is no presumption that the Yavam is more fertile) but rather that there should be a Bi'ah, as this too commemorates the marriage which his brother had.
Thus, after the first Bi'ah the Yavam is entitled to divorce her as he has performed the Mitzvah.
However, there are Rishonim who learn otherwise; see Riva (Yevamos 20a, DH Yavo), who implies that the Bi'ah should be one which can produce children. It is unclear how he will deal with Yibum for an older woman or one who is sterile.
Yoel Domb