Cap cost reductions are taxed up front in CA, so you’ll pay sales tax on the trade in credit value up front just like if you were paying cash. The only way around that would be to have the dealer give you way less than the trade is worth and then give you a huge dealer discount on the new vehicle.
In CA, on a lease buy out, tax is applied to the buy out price. If you’re at the end of the lease, that’s the residual value.
Seems like you are confusing a buyout vs a new purchase. With the buyout, there’s no negotiation or incentives. You will pay taxes on the buyout price.
But to answer the other part of your question, yes you can trade in your lease and apply any equity (if any) as a down payment to reduce loan size or outright purchase amount with taxes and fees.
Essentially when you do the lease buyout from the dealer, it is like you’re buying a used car off their lot, but in this case the used car is the car you leased.
You won’t be able to trade-in doing the buy out from VCFS.
If you have a trade-in I would suggest going through a dealer to facilitate that transaction.
If you want to stick with buying out from VCFS only, then you would just have to sell your other car 3rd-Party. And since there’s no tax benefit/savings you’re probably better off doing this if the dealership doesn’t give you a strong offer on your trade-in.