You might look at using FallbackValue http://msdn.microsoft.com/en-us/library/system.windows.data.bindingbase.fallbackvalue(v=vs.110).aspx in conjunction with TargetNullValue.
In order for TargetNullValue to work you must be able to evaluate the full path of [someViewModel].SomeVar.SomeSubVar.Name to null. If 'someViewModel', 'SomeVar', or 'SomeSubVar' are null then 'Name' can never be evaluated, and TargetNullValue won't apply.