Visio's XML import/export functionality is, I think, just shape related. From an ORM perspective, this is stuff like the page it is one, the X-Y coordinates, etc.
Much of the meta-data in the ORM model is not saved in the Visio XML. Further, I'm not sure you can import it and actually store the meta-data that is exported (mainly, object type names and predicate readings) into the VEA ORM engine. I suspect you would just get a "dumb" drawing.
If the toolbox were to support round-tripping, I'd likely include the (positional) data in Visio's XML in the toolbox XML as well and put it all in one place and with one import function. However, it isn't there yet.
Scot.