I Have no idea how to work thease values. They claim to be vectors, but the seam to affect rotaion rather than simple positioning. Trying to figure them out via trial and error has proven to be a real headache .
Could anyony help me out and explain how they work?

Usually you don't need to bother with the Y offset of them, just the X. Set the ContractedOffset to the shortest distance you want the foot to be from where the leg attaches (enough so that the leg looks natually contracted), so perhaps (5,0), and the ExtendedOffset to the longest distance """"", so perhaps (15,0).
You can play with the Y values if you want the foot to be slightly ahead or behind of the leg, but most of the time the Y values are less than 10.