The same problem happened to me with VW2009 after installing the VW2011 US trial version and updating to SP1.
When I tried to switch to another drawing via the window (or view, not sure about the name in the English version) menu it makes no difference which drawing I selected, it always stayed in the active drawing.
There were other issues with VW2009 as well after installing the VW2011 US trial version, so I deinstalled the trial.
The problem with switching windows remained, and even removing all folders, files and registry entries I could locate of both VW2011 trial and VW2009 and reinstalling VW2009 did not solve the problem.
If I tile the documents, then I can switch between document windows by clicking in the window of the document that I want to become active, but still not through the window (or view) menu.