The VBP Make VS.NET/2005 actions key success/failure off of the exit code of devenv/msbuild, so it appears to be returning 0 (success) here, which sounds like a (VS) bug if any project(s) failed to build.
You could try checking 'Parse build output for failed projects' on the Project/Solution, which will parse the output and fail if ", 0 failed" is not found (but this only works with devenv output and not msbuild).
Alternatively, you could parse the output in the step's
vbld_StepDone script event and fail the step if "<non-0> Error(s)" was found.