I can't find a way to make it work without using a separate element for vab:ValidatorRule and specifying the SourceType attribute.
I do think it's possible to create your own custom validator rule but if it were up to me, I'd rather use an existing one which works rather than creating a new implementation.
By the way, the one I posted above is incorrect, supposing your parent object is of type Foo and it has a property named Person1 of type Person. The correct configuration would be:
<Binding Path="Person1" UpdateSourceTrigger="PropertyChanged">
On the topic of the difference between the standardx wpf and entlib's wpf validation, I haven't really used the WPF built in validation but from what I have read, WPF has only one built in validation rule which is the ExceptionValidationRule. If you
want other validation logic, you would have to implement the ValidationRule class. With entlib, it comes with a lot of built-in validators which you can use and specify either in configuration or through attributes.
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