Hi Cliffmarcum,
Welcome to Vista Forums!
I'm not certain how you are deploying the GPO's, but the following article may help with procedures, guidelines, hints, troubleshooting tips, and so forth:
Deploying Group Policy Using Windows Vista.
On a computer that isn't working, go to Start / All Programs / Accessories / right click on Command prompt and click on Run as Administrator. Type gpresult /v and enter. This should provide a great deal of information about how GP is affecting that machine (and possibly why it is not affecting that machine). Now login as that user on a computer that is working and do exactly the same thing. Compare the two results. I'd be willing to bet SOMETHING is different that's causing the problem.
The Event Viewer has a wealth of information regarding Group Policy. Unfortunately, it requires you to look at all of the different log files to find entries for Group Policy. There you'll find entries related to policy application, policy replication, and policy refresh, all of which can be useful when trying to track down a problem. There is not always a lot of information on specific Group Policy errors in the event logs, but remember that you can always search TechNet if you find errors you can't identify (or post them here and we'll see if we can help). The following article may help here:
Troubleshooting Group Policy Using Event Logs.
(From: http://technet.microsoft.com/en-us/magazine/2007.02.troubleshooting.aspx - which may contain additional helpful information.) Target Object Must Be in the Path of the GPO When you notice that a GPO setting is not affecting an object as it should, there is one more important setting-the object must be in the Scope of Management (SOM) of the GPO. This means that the object must be located under the node where the GPO is linked (even a child node will be sufficient). For example, none of the objects in the Marketing OU will be affected by a GPO that is linked to the Finance OU, as shown in
Figure 3. The SOM of a GPO is from the node where it is linked, down through the Active Directory structure.
Figure 3
When OUs Are at the Same Level, GPOs Only Apply to the OU Where It Is Linked. I mention this because if these computers are looking to a different server instead of the new server (perhaps a different OU), they may not be able to receive the GPO.
I don't know why disabling re-direction doesn't help except perhaps it is only disabling the redirection process and not whatever is causing the computer to want to redirect or even perceive the old server. I feel certain that this is the root cause of the problem - but can't explain why it only applies to half of the computers as one would expect this to apply to all or none - and yet, if it is a difference between those that work and those that don't, it's pretty clearly the culprit.
Not knowing your setup, it's hard to suggest alternatives; however, if possible and reasonable (e.g., you use imaged machines and no user data is stored on them and everything is handled through the network including the logon and so forth), I would consider re-imaging those 15 machines to get rid of whatever is causing them to see the old server (make them like the computers that are working) and then apply the GPO and everything else should work fine on them like it does on the other computers. But your setup could be more complicated than that, so it's hard to see how much effort this would take - but I'd be willing to bet that this would solve the problem.
I hope this helps.
Good luck!