There is some anecdotal evidence that snow evaporates on the cone and doesn't make it down to the tipping buckets in some circumstances. I have seen this happen personally several times with light snow. It shows zero, when there has been measurable snow. I am not surprised that it is lower on the heated VP2, I see the same thing on mine.
As far as 24/7 heater operation, I have a temperature controller that activates the transformer at 2°C. That way the heater isn't running all the time, especially when it is not necessary.
I make no "corrections", but report the VP2 amount in the comments section in the CoCoRaHS data submission page.
Greg H.