Well, the POD is that he isn't.
It's a good WI! FDR managed to edge his rivals aside by cutting a deal with Garner; the resulting ticked balanced a northern liberal with a southern conservative, a formula the democrats would return to later with Stevenson/Kefauver, JFK/LBJ, and Dukakis/Bentsen. (And Carter/Mondale, in reverse; people forget that Carter ran as a moderate-to-conservative "New Democrat").
Now, absent Garner, there are plenty of southern conservatives FDR could pick. But if Garner dies just before the convention gets under way... well, things get gnarly.
Let's see. OTL, Garner controlled a modest but crucial swing block of delegates -- basically, Texas and California. (Which were a lot smaller back then.) He couldn't win, but he could be kingmaker between FDR and Al Smith. OTL, he used that leverage to take the VP spot.
Without him... well, Smith can't win. He lost too badly in 1928; and also, too many delegates have come to hate his guts. (Long story.) But Smith may be able to block FDR until the weary party, after many votes, turns to a compromise candidate; remember, Democratic conventions required a 2/3 vote to nominate back then. (FDR, having run its gauntlet, would spike the 2/3 rule at the very next convention.)
So the question then becomes, without Garner, can FDR still scrape together 2/3 of the total votes. Hum.
Doug M.