This is fun.
the reason why linking is failing for some projects in x64 mode is:
the SolutionDir (or maybe current dir) is implicitly part of the library search path. Therefore, when looking for python33_d.lib, it will find the one in PCbuild, and look into that, and ignore the one in PCbuidl\amd64.
Some projects have "references" to the pythoncore project. This is a .NET feature and not really useful for normal projects, but it does put an explicit link referrence to the correct .lib file into the linker's input arguments. This is why _testcapi works, and not _testbuffer (try diffing the two .vcxproj files).
All of this stems from my patch removing explicit link references to the correct .lib file. Perhaps I can reintroduce that, but in a .props file.