This is one of those PODs that are technically possible but unlikely.
For various reasons, having to do with his age, already having a strong position in DC, and positioning within the party, Bentsen is unlikely to run for the nomination himself.
You get VP candidates that could plausibly have made a run for the nomination last year, if they didn't actually do it, like Kefauver, Johnson, Mondale, Bush, Kemp, Edwards, and maybe Biden. And they you have VP candidates who would have been unlikely presidential candidates for one reason or the other, which is more the norm. Kemp in the other thread you could actually see running himself.
That leaves Dukakis getting into a bad accident in the tank.
Though possible, one interesting thing about all the presidential elections the US has had that the country gotten close to a major party candidate dying during the campaign twice. Once was Teddy Roosevelt in 1912, and you can argue he wasn't a major party candidate, and once was Greeley in 1872. This is not counting something happening during the nomination contest. But thinking it over, its possible. Its hard to imagine how the public would react.
So yeah, if the POD is that Dukakis' tank crashes, Bentsen benefits from a sympathy vote, plus the fact that he is not Dukakis. And he has beaten GHW Bush in an election before. So he would probably win the election.