Unable to compile vdproj with devenv - 'targeting 'x64' is not compatible with the project's target platform 'x86''

You have to mark the Setup project as a 64-bit installer. Your Setup project's TargetPlatform property in the Properties window, change it from x86 (the default) to x64.
Also make sure you deploy the 64-bit version of log4net and that your C# project's TargetPlatform setting is Any CPU.
And make sure that you actually need your app to run as a 64-bit process, that's rare.

1. Open Deployment.vdproj, in Notepad.
2. Find string "TargetPlatform" = "3:0"
3. Change to "TargetPlatform" = "3:1" for AMD64, or "TargetPlatform" = "3:2" for Itanium.
4. Save Deployment.vdproj. In Visual Studio you don't see any changes, but your Project now AMD64.

1. Open a deployment project.
2. In the Solution Explorer, select the deployment project.
3. In the Properties window, select the TargetPlatform property.
4. Choose either Itanium for an Intel Itanium 64-bit platform, or x64 for any other 64-bit platform (such as AMD64 and EM64T instruction sets).
5. At installation time, an error will be raised and installation will be halted if the target computer is not compatible with the specified platform.