UPDATE: As of 2017, this was addressed in Disney canon novel "Star Wars: From A Certain Point of View" (a collection of short stories). In "Added Muscle" story by Paul Dini, we learn that it was a blowback from an earlier episode when Fett crisped some rebel spies and Vader didn't like that fact and refused to pay:
... I picked up trooper buzz that Vader was looking for a couple of runaway droids. Figured I’d collect the bounty and square myself with the headman at the same time. He’s still got a mad on over those rebel spies I crisped on Coruscant. Idiots came at me with ion disruptors. What, they thought I wouldn’t carry a weapon accelerator? Flash, boom, three tiny ash piles. Tried to collect and Lord “No Disintegrations!” refused to pay without bodies. My word’s not good enough, apparently. ...
... Betting the third ran with the droids. I’ll hunt around after I’m done here. Vader may triple the bounty if I bring him the fugitive along with the droids. Yeah, I know, intact corpse, “no disintegrations.”
The clarification was because the "dead" part of "dead or alive" would be faster and easier for Fett, and therefore without specific instructions that the bounty should be alive he'd go for the "dead" option.
"disintegration" was just a figure of speech - Fett would have likely just blown up the Falcon, which would presumably disintegrate the bodies.