My understanding is that the 3.5 is the 3.2 block bored to a larger size - that's why the intake plenum is interchangeable. To put the direct injected engine in the VX it would take more than the motor and computer - I believe there's other stuff to replace too - it's basically an entirely different under-hood setup (drive-by-wire throttle - not sure about that but the harness and cableing look different). That same ECU also manages the Axiom's stability control system (missing on the VX - it was offered on the Axiom, Rodeo and Trooper starting in 2002) - I'm not sure what the absence of those control systems would do in a ECU swap (maybe nothing), but it could potentially throw codes if it can't find that part of the harness.
-- John