A few of my friends have bought cars from Carmax down South. From what I have heard they are very reputable people to deal with. Take it back and have them repair the problem the right way. The VX has a 30 day warranty which should cover all the repairs, after all I am sure Carmax checked out the VX before they put it up for sale, if it was bad to begin with they would have wholesaled it and not bothered to put it on the lot.
If a new motor is called for go for the DI.
John