This is written without any specific knowledge about this unit other than what I've read so far  .. but that has never stopped me before 
=> it 
looks like this unit has a Variable Input Impedance  [very excellent !!]
=> it 
looks like it cannot Receive  -
and-  Send  Midi CC  (?)  [not good]
Other than the L6 HXFX having a fixed 1M ohm Input, Line 6 HXFX seems to have and do 
way more (?) .... plus it has 9 Blocks, scribble scripts, more FSW's, full Midi spec, hosts full spec IR's, has the full suite of the full Helix efx ... etc....
I'm really struggling  to see [
given the current very limited info]  why a potential buyer would pick the VP4 over the Line 6 HXFX (?)