And now I even found the solution:

$SystrayWidth$ is ignored completely and is _not_ exported by vtray-x.x.x. It is in fact apparently only a "leftover" from the time when the Austerity-Theme was developed. And it is very well suited to really mix up any one who wants to change those sections of the theme :-)

So here is how it really works: The taskbarwidth is adjusted in script.rc, using the bangs TaskbarShrink, TaskbarSize and TaskbarGrow.

These script-lines need to be replaced:
TaskbarShrink by xTaskbarResizeBy (with minus-sign before the increment)

TaskbarSize by xTaskbarResize

TaskbarGrow also by xTaskbarResizeBy (no minus-sign before the increment)