So I finally got some time today to setup a Windows 7 VM, Visual Studio 2012 and debug the C# console application testcase provided in comment 18. I can confirm that it's indeed broken with VirtualBox 4.2.x and even with single VCPU VMs and is a VirtualBox bug.
The new VT-x code in trunk doesn't have this problem. I didn't have time to investigate even deeper. Unfortunately, identifying and backporting the exact fix to the 4.2.x branch will not be trivial and will require more investigation effort and time. All I can say now is, it should be fixed in the next major release, not in 4.2.x.
Maybe later we could provide you with a test build here for you to try that the fix works. Thank you for the report.