How big a fuse do I need?

For heating, lighting and electronics, you want the smallest fuse that is more than 10% over the rated current of your product.   The rated current is the wattage divided by 12, in 12V systems.  So for example, if your vest is 45W, it draws 45/12 = 3.75A.  10% more than 3.75 is 1.1 * 3.75 = 4.125.  Use a 5A fuse.  If the manufacturer supplied or recommended a fuse size, do what they do.  For motorized devices, you may need to go higher to handle startup surges.  For double outlets, size to fuse to the nearest size larger than the sum of the currents of  the accessories you expect to connect.