No need to change anything on the 8924 once Johnson's firmware installed. 8924 Bridge mode in Broadband Settings. VLAN to 101 in modem.
PPPoE handled by Router. Modem will 1508 without intervention. SET ROUTER MTU to 1500. Worked for me. Incidentally, I have found the 3927 to have less upstream errors (ES) and 3MBs better DS sync than the 8924 on my short but noisy and poorly installed 265m BT FTTC line, but that can vary line by line as I'm sure that you know. I've an 8924, 3925, 3927 and a 1312 that I'm trying on 14day cycles at the moment after downgrading from G.fast due ££!
Edit: PPPoE by/from router and straight through modem for BT FTTC VDSL2, sorry for the previous assumption! I just took delivery of a VMG3926-B10A as well, nearly a full set to keep me going. Johnson's firmware is a real Godsend, as is this treasure trove of information that is Kitz.