Bangle Js v2

I guess that the vcare library is not open source?
Does the library also compute HR?

Typically there are two separate parts, the low level driver stuff that can turn on/off the led(s), setup the sensitivity, detect proximity (=you are wearing it)
and once you get stream of data there is higher level heartrate algorithm library you feed the data to that gives you heart rate.

The lower level part is not a problem to have open source, the information comes from datasheet or sample code available or reverse engineering. We found enough of this for VC31 (well, except the datasheet).

The higher level comes as static library that can be linked to the project and is not opensource. This is of course not usable for us and is probably even not worth it.

Previously I had same experience with PAH8001 (DS-D6) and HRS3300 (many other watches) - same structure, very similar library where you feed sensor data and accelerometer x,y,z periodically and after some time it starts giving you heartrate.

As for VC31 the manufacturer page is http://www.vcaresensor.com/ and there was even VC31 subpage some time ago but it gives 404 now. When I tried to contact them (months ago) for more information via their contact email I got no reply. Fortunately someone published enough information on github as part of his hardware project that includes this sensor. There is even the high level closed source library. But still, this can be used just as a reference. Not sure if the library would even link correctly with nrf52 build.