|As you put it, there is no solution as yet but a workaround and there seems to be no solution that will be developed. In fact there are two workarounds if you have a version of Vista that has Bitlocker utility.|
One of the workarounds is:
In XP, just install TweakUIXP from Microsoft, then start TweakUIXP and expand My Computer, click Drives and remove the checkmark from checkbox of the drive where Vista is installed.
This will blind XP from seeing Vista partition and erasing Vista restore points.
The other solution is:
In Vista, enable Bitlocker.
Hope the above helps.