I would disable value insertion in the MaskedTextBox, but keep the validator for that field.
I use kendo with MVVM.
This is my html code:
<div>
<label for="elem" class="elem">Field</label>
<input data-role="maskedtextbox" data-bind="value: elem1" id="elem" name="element" class="form-control" placeholder="Insert" required validationMessage="Insert {0}" disabled/>
<span class="k-invalid-msg" data-for="element"></span>
</div>