The most likely reason for that is that the new one has the latest updates and service pack which the instalation interprets as a newer version.
Creating a new Vista disk with the latest service pack by slipstreaming them onto the Vista disk could work.
Here is one tutorial on it, you can Goole for more information.http://www.vistax64.com/tutorials/230249-sliptream-vista-sp2.html
and another: http://blogs.techrepublic.com.com/window-on-windows/?p=1249
Or wait and see if someone can come up with an easier solution.