I mentioned some of the issues I had with .vdx files created in Visio 2010 Beta and opening them with RTM here. It seems that you can actually modify them manually to fix this error by simply using any text editor to open the .vdx in question and change...
xmlns:v14='http://schemas.microsoft.com/office/visio/2009/7/extension'
...to...
xmlns:v14='http://schemas.microsoft.com/office/visio/2010/extension'
...on the first line and it then opens in RTM fine. Only tried this on a few files, but so far seems to work fine.