The lowest energy state of a collective system may often find itself in an energetically unfavorable local minimum. We can estimate the rate of formation of bubbles of true ground state, by reducing the problem to that of quantum tunneling for a single collective variable, namely the bubble radius. This is called the 'thin wall approximation'. We have studied systems in which the true vacuum is enclosed in the interiors of topological defects which are themselves stable, but can act as seeds for catalyzing the formation of bubbles of true vacuum. Using symmetry arguments and suitable limits of parameters we are able to identify the appropriate collective variable and apply the thin wall approximation to two examples, a 2+1 dimensional vortex and a 3+1 dimensional cosmic string. In certain limits of the parameters, the putative metastable state may in fact be rendered unstable, causing a 'roll over' of the system to true vacuum without recourse to any tunneling.