Just to add to this, my new VDSL2 connection, for which the BT Wholesale broadband checker reports clean speed estimates 56.8 - 40 Mbps download and 11.0 - 7.7 Mbps upload, has the following stats:
Stats recorded 18 Dec 2020 12:09:53
DSLAM type / SW version: BDCM:0xc190 (193.144) / v0xc190
Modem/router firmware: AnnexA version - A2pv6F039v2.d26a
DSL mode: VDSL2 Profile 17a
Status: Showtime
Uptime: 15 hours 33 min 23 sec
Resyncs: 0 (since 18 Dec 2020 12:09:49)
Downstream Upstream
Line attenuation (dB): 21.3 0.0
Signal attenuation (dB): Not available on VDSL2
Connection speed (kbps): 40000 9997
SNR margin (dB): 13.4 6.7
Power (dBm): 11.9 7.4
Interleave depth: 16 1
INP: 45.00 0
G.INP: Enabled Not enabled
Vectoring status: 1 (VECT_FULL)
RSCorr/RS (%): 0.0001 0.0025
RSUnCorr/RS (%): 0.0000 0.0000
ES/hour: 0 0
As you can see, I have vectoring and am using the A2pv6F039
v2 DSL firmware on my Zyxel VMG8924-B10A. I have none of the sync speed halving (if this was happening I'd expect to see perhaps 30 Mbps on my 40 Mbps capped connection (I only subscribe to the 40/10 product) as the maximum predicted speed is 56.8 Mbps and my reported max attainable speed is 60.5 Mbps) or other strange behaviour that is observed for the A2pv6F039
v firmware. This means I now join GigabitEthernet in having success with the Openreach approved
v2 DSL firmware on a vectoring enabled circuit.
Also of note is that I have INP = 45 and INPRein = 0, suggesting RetX low unlike GigabitEthernet here
https://forum.kitz.co.uk/index.php/topic,22086.msg426585.html#msg426585 who can't get rid of RetX high (though this may be because of my 40/10 cap).