A guidewire assembly for use in deploying a bifurcated endoluminal vascular prosthesis that has a main graft portion and at least a first branch graft portion The guidewire assembly include a hollow guidewire sheath having a restraint mechanism, such as a tubular sheath, for constraining a branch graft portion of the vascular prosthesis and an inner core wire that is slidably insertable into a central lumen of the hollow guidewire sheath. In use, the guidewire assembly may be used with a deployment catheter to deploy the bifurcated vascular prosthesis and leave the inner core wire in position in the patient's aorta, extending through the main graft portion of the vascular prosthesis.