In case it's of any interest, there's a publicly-viewable section on
VP/VP2 rain gauge calibration and faults on a page in our VP
knowledgebase. See:

http://vp-kb.wikispaces.com/Sensor+Errors3

I'll be adding another paragraph to this, probably in the next few
days (if I get time), dealing with height vs calibration issues and
the fact that the drip test - good though it is - won't necessarily
give a calibration result that's exactly in line with a ground-mounted
reference gauge.

John Dann
www.weatherstations.co.uk