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.