You can try making your view model of type IValidatableObject and do a custom validaton

public class User: IValidatableObject
{
    [DisplayName("User Name")]
    public string UserName { get; set; }

    public IEnumerable<ValidationResult> Validate( ValidationContext context )
    {
         if (Conditions))
         {
              yield return new ValidationResult("Error Message" );
         }
    }
}