As I understand there are two causes of vampire drain: real drain which is actually energy used to support some functions like bluetooth, WiFi connection and so on and not a real drain, but loosing some capacity over the time. Even if you completely disconnect the battery, it still will lose some charge over the time (which we cannot call a drain). Also dc2dc converter from 400V to 12V works constantly, does not matter car is in a sleeping mode or not. That converter also has some losses.
When you drive day time charging the 12V battery does not take much higher energy, probably most of it goes to the screen and some pumps. I guess AC takes energy from high voltage battery directly.
So, back to your question. I thought about it. I think that power drained by 12V battery is probably a biggest part of all "vampire drain". If it is, yes, connecting your 12V battery to some charger (solar panels) will help. But from another point, charging 12V battery is still not free generally. Also, dc2dc converter will still connected, and have some loses even with no load. If car left for long time, probably it worth to try and see what happened.
I think in a future Tesla will get rid of 12V battery completely and will use more than one dc2dc converters.