hi Bhushan,
thanks for your suggestions. I reinstalled CR Runtime 13.0.8.1216 for 64bit computer, recompiled and republished my app, but Im still having problems in running reports on 64bit users' computers.
Let me point out the steps I did to deploy my app.
First, notice that my MVVM application consists of a 3 projects solution - say AppModel, AppViewModel and AppTest.
I set target CPU to "x86" and target framework to ".NET Framework 4" for each project.
Then I added Microsoft .NETFramework 4 and SAP Crystal Reports Runtime Engine for .NET
Framework as prerequisites for deployment.
Finally, I included all CR Runtime DLL's (ie all files under C:\Program Files (x86)\SAP BusinessObjects\Crystal Reports for .NET Framework 4.0\Common\SAP BusinessObjects Enterprise XI 4.0\win32_x86\dotnet folder) in AppView project, and set the "copyLocal" option to True, so that the DLL's are included in the deployment package.
Now Im able to install the correct CR Runtime (32 or 64bit) in the user's computer. Reports are correctly loaded when I run the app in a 32bit computer, but not in 64bit computer. The more strange thing is that
I can load reports also in my development computer (64bit OS)! Running a report on a "clean" 64bit computer causes this error message (note that the error is raised by AppViewModel project). 
I also tried to set target CPU to "x64", but this causes app not running at all!
Im struggling with it! What is the correct CRR to be used?