Boats with no vang whatsoever (neither centreline nor toerail) are prone to rolling when sailing deep angles (broad reaching or running), because the flow across the top panels
of the mainsail
can easily reverse in a rhythmical way.
Another way of saying this is that the top of the main lies in line with the plane in which the boat rolls, so it does not provide any roll damping.
Because it is so far from the centre of roll, that is the location where damping/stabilising would be most effective.
Furthermore, the sail will 'pant', with the boom rising and falling, the sailcloth chafing on the rigging
wires (particularly at the battens), and the changing sailshape accentuating the variable forces, contributing to the rolling.
Does not apply to the OP, who inherited a toe-rail vang, but I thought it worth mentioning for the benefit of those wondering about the question in the title.