Hi xpmaniac ,
Welcome to Microsoft Windows Vista Forums.
As you mentioned that you are able to install Windows vista 32 bit using the 32 DVD.
May be that the 64bit DVD has gone bad or has some scratches on it.
Clean the surface of the 64 DVD with a clean cloth and try the installation again,if that fails please get the 64 bit DVD replaced,might be the way the disk has been burned might be incorrect.
Hope this information is helpful.
Let me know if this worked.
All the best!
Thanks and Regards:
Halima S - Microsoft Support.