Joel,
An absolutely clean surface is critical to any paint job. Use a metal primer paint first and let dry. Then apply a few coats of color enamel. Let dry.
Just a note on spray can enamels. These enamels dry to the touch in a short order, but they take for ever to fully dry. I generally give things a full month before pressing them back in service. You can speed up the drying process with heat lamps or bake painted parts in your oven at low temperatures (say 250 degrees) for several hours, then turn the oven off and let the vise cool down in the oven overnight. In the morning your vise will be cool and the paint will be hard, dry, and durable.
Note: Your house will stink if you use the oven and your wife will be pissed at you long after the smell is gone.