Hi Kristoffer
A lot of Vector GPS systems are in use in the UK with Sontek M9 ADCPs.
We mostly have the V102 (this is what I have), and it seems to work really well. The V102 is not super high precision, but is certainly good enough for routine discharge measurements.
It could be argued that you might want to calibrate your compass in case of issues with the GPS, but as long as the GPS signal is good enough (which it has always been for me in the UK), the need to calibrate compass is pretty much eliminated.
I believe the cable that enables the V102 to be added to M9 ADCPs was developed by Nick Martin at Xylem in the UK.
One thing to note: the battery consumption increases quite a lot.
I’m afraid I can’t advise if you want higher precision, but for us the V102 has been great!