How did big Pterosaurs get off the ground?

[Bowhead Whale]

Agreed.
The vampire bats and ducks are only there to provide "insight", a layman's visual aid of sorts. No one's really claiming that because microbats quad launch when grounded, then pterosaurs must have too. (though it's interesting that people easily assume that pterosaurs launched like birds just because)

I linked the 2008 paper because it details the reasons why quad launching is possible for pterosaurs, but it doesn't say that they definitely did. While a bipedal launch is not impossible for the tiny pterosaurs, the giants would have needed a different strategy since the biomechanics of their hind limbs prevent them from relying on a bipedal launch (whether leaping or running).

I will concede though, unless we find trackways that show a "takeoff", all we'll ever have are the computations.

Pterosaurs couldn't indeed take off like the highest number of birds when you know pterosaurs on land looked more like apes with their long arms and short legs than like birds. I think, like many poeple here, that the vampire bat provides us the answer.