Arent President and Vice President now elected as a team meaning that Cheney has as many electoral votes as Bush. I know in the early days that was not true and we had cases where the President and Vice President were from different parties.
I cant imagine anyone standing for Bush winning the election and then because he was killed, the opposition party took office. No way that would fly, especially if it wasnt close.
I suppose it could happen if the president and vice president were both out of action and the speaker of the house was from the opposition party.
If it happened after the election, but before the inauguration and Bush and Cheney where reelected then Cheney would be president for the next four years.