Ah, that's an auto-generated header from a class dump or something like that. I was looking at the "real" headers from the iOS SDK, which use vector types. (The iOS SDK headers aren't available online, the closest thing you can see is Apple's online documentation for GKAgent2D.position.) I'm guessing that during compilation some "magic" happens to properties/methods that use vector types, and the result is what you see in the class dump header. You could try using the methods position_ and setPosition__ (note the extra underscores), which according to the class dump header use structs instead of vector types.
Thank you very much, using the methods with underscore solved the problem!