I understand what you are trying to do but it seems you need to be forewarned about a few thing. Here are my reasons; Vehicle A will be your car and Vehicle B will be the one you wish to purchase.
For the sake of numbers, lets say Vehicle A and B both have a price of $20,000. Vehicle A has a lien and Vehicle B is owned outright.
Issues for Vehicle A:
1. Vehicle A is being sold for $20,000, but how much is it WORTH? $18,000? $15,000? Vehicle A needs to be appraised. In today's used car market it's all about value. I would recommend going to a VW Dealership, CarMax(if one is local to you), and another random Dealer to get some written quotes. Ask each Used Car Manager or Appraiser what they rate the vehicle's condition as. They will use the words Excellent, Very Good, Good, or Fair.
2. How much is OWED on Vehicle A? Vehicle A might be going for $20,000, but that's because that's the remainder of the loan, but if it's worth only say, $16,850 a prospective buyer isn't going to pay much more than that. Do you have the money to cover the gap?
Issues for Vehicle B:
1. Again, how much is it WORTH? Don't make a poor investment by overpaying.
2. Tax, Tag, and Title. Since you're purchasing from a private party you will have to handle this all on your own. Some states let you list the bill of sale for $1, but if you have pay Personal Property Taxes most likely they will come and appraise the vehicle for tax purposes. Make sure you have the money to cover these expenses.
3. Financing. Unless you have the money in cash you will need to get approved for the purchase of the vehicle. Again don't make a poor investment if the rate and or payments are too high.