Vaillant inbuilt monitoring vs MID meters

0.1K is excellent, so any COP issues would seem to be real (unless theres an issue with the flow rate sensor which is the second measurement entering here).

If you’re a bit technically inclined you can get an ebus adapter to log all those internal measurements. Not as accurate as a full heat metering setup but cheaper and will give you a great start. There’s a couple of threads on that here as well, e.g. Vaillant eBUS hardware adapter (ebusd software) Thread