xDSL is a very mysterious entity.
Your ADSL2+ circuit has a metallic pathway from you, via the PCP, to the exchange building.
Your VDSL2 circuit has a metallic pathway from you to the cabinet based DSLAM, via the tie-cables to/from the PCP.
There is a longer length of cable for the former when compare with the latter. Once the two circuits have left your home, they may actually transit in different D-side cables to the PCP. It may be that your ADSL2+ circuit is "close" to another circuit radiating "noise" within the cable.