I made a programm to use that undocumented
Vxd Call to reserv some shared memory
and hooked the VxD call. But when my
VxDCAll is called, I still can't modify
the adress in System Area, such as
Kernel32!GetLocalTime etc. How do I modify
such adress??? I thought under Ring0 you
can do everything to all the memory.

You CAN do what you want when in Ring0, provided that the page you are writing to is present.
So, it would seem that the problem lies somewhere in your VxD or in the communication between the program and the VxD.
You could upload both program and VxD, see if somewhere has the time to take a look, if you haven't found the prob yet yourself.

Blue skies
Fake

I found out how to do that in Ring3
mode, Ring0 is not neccessary to do
such simple work

Use the VxDCall PageModifyPermission
to do anything you want