The version of Vista matters.
"On a system running x64 Vista Home Basic, you can add as much as 8 GB of RAM. x64 Vista Home Premium supports as far as 16 GB of RAM."
Before Vista can report that much, the BIOS must show it first. And then some folk forgot this nugget.
Read http://forums.techguy.org/windows-7/929591-solved-msconfig-max-memory-problem.html for another limit.
I have 24 gb of memory installed, but Windows Vista 64 says I only have 12. I removed all 6 of the RAM and reinstalled, reversing the order to see if I had any defective RAM. It still shows only 12 gb. So I am assuming I do not have defective RAM. How should I trouble shoot this?
Mushkin Enhanced Blackline 6 x 4 gb
Note: I have another identical workstation with no problem.