The sln and vcproj file formats can change between Visual Studio versions, so the only reliable way to do conversions is with Visual Studio and the Qt Add-In for Visual Studio.
Once the Add-In is installed, open your Qt solution with Visual Studio and select the “Create basic pro file” option from the “Qt” menu item. The Add-In will generate pro and pri files with all the relevant settings from the sln and vcproj files.
If you want to open pro files from Visual Studio instead, just select “Open Qt pro file” from the “Qt” menu and the sln and vcproj files will be created automatically.
Do expect to make minor adjustments to the generated files, as the conversion is not perfect.
Note: This doesn’t work with the VS Express edition, since that version doesn’t support plugins.