For those of you who had the same problem I did.
The Flying Vader is actually a pretty crappy plane, and it doesn’t even nosedive like in the book. Tom says it’s not really meant to fly, but you can tweak it and get something decent out of it.
What to do:
Step one: Tape. Take one strip of tape, cut it in half, and apply to both sides of the place where you grip the plane to throw.
Step two: when the book says adjust the dihedrals, it means fold them up. Fold them WAY up, and that will help Vader fly.
Possible flight scenarios:
Does a perfect nosedive like in the book.
Flies a few feet before dipping and crashing, possibly spinning while in flight.
Your Vader should fly according to the first two scenarios, so while it won’t outfly a Nakamura Lock like Harvey says, it will actually fly quite well and maybe remind you of some other planes you’ve folded. Hope this helps! Do not throw in windy areas, that screws up your flights although it does make for smooth landings. And of course, do not throw in a museum of priceless historical artifacts!