Skunk.....In my former life I was an assistant lease manager at a Honda dealership for 7 years so I have some good knowledge about leasing.
Your remaining balance is not affected by Kelley blue book, just like with a conventional loan...say through your credit union or whatever bank you might use...the difference between what you still owe on the car and what the dealer will give you for it is your responsibility (i.e. if the balance on your loan is 15000.00 and the dealership is giving you 12000.00 for your VX then you are responsible for that 3000.00 difference...how you handle that difference is up to you...you may "roll it into the next purchase" up to a certain dollar figure depending upon your credit....you may offset some or all of the inequity with a down payment....or you can sell the vehicle yourself to try to break even.
In Texas the one advantage of trading it in with the dealer is you only pay sales tax on the difference between the new car minus the value of your trade in (new car 24000.00 - 12000.00 for your old car = tax on only 12000.00 instead of 24000.00) check with your local tax office and they can tell you.
YOU CAN TRADE YOUR LEASED VEHICLE ANYTIME YOU WANT - the longer you keep it though the better position you will be in come trade in time, as is true for a conventional car loan. Typically if you have a 4 year lease you should stay in it for at least 3 years so as to reduce your inequity.
Leasing is not BAAAADDD if you meet the parameters that leasing what designed for....for alot of people a conventional loan is a much better choice depending on their driving habits and financial situation.
Don't let anyone tell you you can't trade in a leased vehicle it happens thoussands of times all over the country each day. If you had done say a 60 month conventional loan on your VX you would still have a balance that was higher than the value of the car and have to make up that difference.
If you need anymore info you can "private message" me and i would be happy to help you out!