VDSL is not compatible in any way with ADSL.
VDSL2 capable line cards (the ISP end) are compatible with ADSL2+ ADSL2 and ADSL - so you can use these modems on a VSDL2+ connection - Its easier to find on this juniper networks document, as well as this broadband-forum presentation - I am unclear if its possible to use a VDSL2 modem with an ADSL connection
ADSL 2+ should work with ADSL2 and ADSL in both situations