As said above, once you adjust your front springs, it will jack your sag. If you adjust your front spring rate you most likely have to adjust your rear also. Rear might have an adjuster tension ring that you can turn a couple times. 15W will slow down the dampening rate, but there is valving in the fork tube also, most of the time the fork dampening is adjusted with valving and amount of fork oil in the fork tube. Being that you have a Vulcan, I don't think you want to spend that kind of money on your suspension. Revalving cost's a few bucks! Keep close track of what you do, write it down so you can easily back track to the last step or back to complete stock, because once you adjust anything with your suspension you might very well mess the whole feel of your bike up, and you might not be happy.
A simple "Hack" I will label it as a hack, its not really a hack on a dirt bike, its just not used on a street bike, but you might already have spacers. You could try a small spacer made out of PVC or metal tube the same diameter as your fork spring, and it will compress your spring a little bit, you can see how you like that before you buy a stiffer spring. Put the small spacer that you make, try 1/4'' to 1/2'' inch right under the fork spring cap and reinstall the spring cap. Remember to properly torque it down! Never add air pressure even if it has a Schrader valve. If this higher spring rate is more to your liking then stiffer/ higher spring rate springs will be what your looking for.
If you already have spacers, just make new spacers 1/2" inch longer and see what you think of that. Also, find the OE fork tube fluid volume, if there is a min/max, drain old fluid and measure, if your near the minimum, ad new fork oil to the max level, that should help with dampening also.
I have used as much as 1" inch spacers on dirt bikes made for permanent spacers.
