Don't feel bad, I got the same manual gauge a few weeks ago and my VP2 is not even close. The Davis is under reporting rain by 70% to 80%, this is with at least 0.15" of rain each day I compared. All the obvious things are not the problem, I just think it wasn't properly calibrated when leaving the factory, adjustment screws were basically screwed all the way down.
I've adjusted the screws, got the buckets tipping with an equal amount (roughly), and the last rain was Davis 0.18", manual 0.20", so it's a lot closer than it was. Going to rain tonight/tomorrow so I'll see how it does after that last reading.