Vinick is a great example of a moderate conservative character of the sort that Will McAvoy is supposed to be. He's pro-choice and non-religious, but he still has solid conservative credentials in other areas, as evidenced by the positions he presented in the debate episode. But as bigdaddy
said, Vinick showed up years after Sorkin left the show, and he was part of an effort by the new writers to challenge The West Wing's (understandable) liberal bias.
Wasn't Vinnick supposed to win, but after McGarry died they felt that was a little too harsh? I've read that somewhere, at least.
There are conflicting stories on the matter. Lawrence O'Donnell, Jr. claims that Vinick was originally supposed to win but when John Spencer died they changed it. John Wells, showrunner and co-writer of the election episode, claims that the Santos victory was in the original script for the episode before John Spencer died and that the changes to the episode were purely to include Leo's death.