In Vectorworks 2011 you could snap to objects that were not on the working plane.…
Vectorworks Tip 346 – 3D Modeling – How To Make an Object That Turns a Corner
This is not the only way to do this (cehck out tip 349 comming later) but it is one way. The challange is to get an object to turn the corner. If you extrude an object, split it, then mirror it, you can create teh object you want.