Gadget & the BMW Riders Webpage
Our Vulcans use dual 1157 bulbs as a combo tail light and brake light. If you'd like a bit more visibility when braking, you can change this bulb out for a 2357 bulb. The 2357 is available at any auto parts store (there's also a 2357A which is yellow). It produces the same brightness for the tail light (3 lumens) but 25% more light for the brake (40 lumens vs. 32).
Gadget Note: Now that it's 2017 you may want to consider upgrading to an LED taillight bulb.
So why didn't Kawasaki use the 2357 bulb in the first place? Well, there's a downside or two that you'll have to weigh.
The power required to run the 2357 bulb is the same as the 1157 in normal running but with brakes applied there is a slight increase in the amps required (2.1 for the 1157, 2.2 for the 2357). The resulting increase in watts used is 1.28 watts, or a 4.76% increase over the standard 1157 bulb. You probably won't lose any sleep over that. The major sticking point is bulb life. Your 1157 brake light element is rated for 1200 hours while the brighter 2357 element is rated for 400 hours. (The tail light element in both bulbs is rated the same at 5000 hours). THIS JUST IN....If you can find them, Sylvania has a long life 2357 bulb labeled 2357LL which has about double the life of the standard 2357.
And the price of 25% more light while braking? Less than three dollars for both bulbs.
There is also a Halogen bulb option but you'll risk melting your tail light lens using it. Details are on this BMW riders webpage (along with more stats on the above change): http://www.ibmwr.org/ktech/taillite.shtml.