The advantage of designing in vectors is that the designs can be blown up to whatever size you like without any loss of quality. The industry standard vector editing software is Adobe Illustrator. I've tried it once or twice many years ago, but never got on with it. My designs were done in CorelDraw, which can normally be picked up at a good price (maybe for the previous version). If you're looking for free software, TechRadar published a recent list here. The only one I've tried is Inkscape, and that's probably changed a bit from when I last used it.