The paper explores London-based third sector practitioners' engagement with vulnerability in their work with refugee and migrant women during pregnancy and in the postnatal period. Practitioners draw on notions of vulnerability that signal weakness and passivity as a strategy, which enables them to secure resources for the women they support as well as to sustain their own organisational existence in a third sector landscape that has been transformed by a range of neoliberal measures. Despite this invoking of essentialised vulnerability practitioners possess an awareness of how the broader context of women's lives, including government policies and structural disadvantage, acts to shape their vulnerability. The paper argues practitioners' contextual understanding of refuge and migrant women's vulnerability resonates with theoretical approaches that conceptualise vulnerability as an ontological characteristic of human existence. Strategic use of essentialised vulnerability is central to accessing resources, while an ontological understanding of vulnerability as a universal potential activated by socially mediated unequal power relations enables practitioners to address the specific factors that are producing women's vulnerability to harm. Crucially, this includes challenging the effects of the UK government's anti-immigrant "hostile environment" policy and neoliberal austerity measures.