I am not aware of a simple way of linking graphics in Visio so that multiple files can share the same source 'page'.
I think what you will need to do is save the entire contents of the background page as a new shape in a stencil. And then place an instance of this shape in all of your Visio documents. Then if you need to update it, you will need to firstly amend the master in the stencil and then use the change shape command to replace the master on the document stencil. This principle is discussed here
Visio 2007 Pro: Update master on Document Stencil from new master? although looking at that thread the user wasn't happy with the size of the shape after updates.