Are modems, made for a more recent DSL standard, downwards compatible with older DSL technology?

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

Actually there are modems that are both ADSL2+ and VDSL2+ capable, such as the Actiontec Q1000 or the Actiontec C1000A. The Actiontec Pk5000 originally was only ADSL2+ capable, but the current Actiontec PK5000 modems are now both ADSL2+ and VDSL2+ capable and Actiontec is working on an update to make their older P5000 modems also VDSL2+ capable.

ADSL2+ technology generally has speed capabilities of up to 20M downstream/896k upstream. VDSL2+ is newer technology and has speed capabilities far greater, such as 40M downstream and upstream speeds of 5M, 10M or faster, instead of the standard 896k speed. In some areas - generally major cities - there are now VDSL2+ speeds of up to 100M downstream/40M upstream available.