May 04, 2012 10:39 AM|zuperboy90|LINK
For this you will probably need to create a custom [UserNameRequired] validation attribute where you get the value of "IsEditMode" and validate based on that.
Here is an example of how to use validation attributes and also how to get the value of another property.
Anyway, this makes it more complicated than having two ViewModels.
Another solution can be to have a base class which contains common properties, and two derived classes, one for AddUser and the other for EditUser.