Our T&Cs state that any overoccupancy will be charged pro-rata for the whole of the stay. In your case, we could charge 50% more.
I am pretty sure that if you tried, without having a specific term in your contract, VRBO would refuse to draw from the deposit.
Even if you had the right wording, unless you have a tiny tank (which is your fault), it may be difficult to link it to over occupation.
As I'm sure you know, a septic tank has a finite capacity and must be pumped out every three to five years depending on usage. As annoying and upsetting as it is to have guests be dishonest and exceed your maximum occupancy limit, you would need to have the septic tank pumped regardless. They may have hastened the need to have it pumped by a week or so. You cannot legitimately charge the guests for the cost of having your septic tank pumped.
It depends what you call a septic tank.
Normally, septic tanks breakdown organic matter into gas and sludge. The sludge has to be emptied every 5 years or more.
Some tanks are not septic but storage tanks and have to be emptied every 2 months.
Having said that, even if you have a storage tank, forget charging the pump out to a guest. It's too close to normal wear and tear that we can never charge to guests.
I have 3 cabins on septic. 1st off, i would check if there is an existing underlying septic issue. 12 although being over capacity should not cause an issue in itself for a short term stay. If so, then maybe even 8 for your septic it pushing it.
I have added laminated signs over each toilet not to flush prohibited items and what associated charges will be if they are found to cause issue. I also have it in my rental agreement that capacity cannot be exceeded because property is on well/septic.
When we built our 3rd property I over-built in terms of well and septic capacity - got a bigger tank than was required and had deeper/more productive well drilled. Knowing that every now and then, despite my best efforts, someone will go over occupancy.
I find that in a rentals that sleep larger number of people you have to over build and over engineer everything almost to commercial grade else it will not hold up.
Hope you get it resolved.