I work for an ISP rolling out G.Vectoring so I know a little about it. While the info above is correct, its missing the point.
The crucial thing is that vectoring requires 100% compliance within the group. The group can be a line card (~30 ports), a rack (192 ports with Huawei gear) or a full DSLAM (n*192). For all cases the lines are all doing computations to reduce crosstalk but they all need to inform the port in the DSLAM what noise they're seeing so that it can "co-ordinate" the negation of said noise. If 1 modem in a group doesn't send back this info the entire group (possible a full housing estate) get reduced performance as that line is an unknown and its noise cant be negated.
You're lucky, some ISPs will kill your port when vectoring is enabled if you're non compliant meaning your get no VDSL signal whatsoever and your connection goes dead. Its then up to you to get a compatible modem for it to be turned back on.
While your modem may say it supports it, there have been issues with certain modems not implementing it correctly. One good example of that is fritzboxes which are performing very poorly with vectoring.
Also, they can tell roughly what you're using. The DSLAM and modem communicated during the sync broker process and the DSLAM is informed of chipset brand ID, MAC, firmware version and serial number. There are possibly a few other bits of info too on the backend but they are the main ones. This is all before you get a PPPoE connection back to the BRAS.
Check for firmware updates from Sagem first of all. G.Vectoring is "bleeding edge" and not all the kinks have been worked out. If that doesn't resolve it then you need to replace it. There's a good change Huawei made the DSLAMs (it's them or Ericsson) so going with their gear isn't a bad bet. AFAIK the 658C only goes to ISPs though. You might find a Zyxel VMG8324-B that'd do it though. If you want more advanced features the Draytek Vigor 28XX's are the ones to go for, but they're expensive.
After you become compliant you may need to call them and ask for the port to be reset and re profiled to 50 (or maybe more with vectoring benefits).