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.