search id of knight armor in items.xml ( it is 2476 ) and then go to movement.xml search for id 2476 and you will find this
HTML:
<movevent type="Equip" itemid="2476" slot="armor" event="function" value="onEquipItem">
<vocation id="4"/>
<vocation id="8" showInDescription="0"/>
<vocation id="3"/>
<vocation id="7" showInDescription="0"/>
</movevent>
and just delete the <vocations> to allow all vocations to use that armor