In theory, the rear shocks should "sag" about 3/4" when you sit on the bike. Achieving this really seems to matter how much you weigh and how worn the shocks are. Many here just fill their shocks with about 10% of their weight in psi, meaning you weigh 180 pounds you put 18 psi in each shock.
Make sure you don't use a high pressure hose and don't over fill (I think max is 40psi, but you should check the manual on this)
As there is no real preload adjustment on the springs, setting the sag can be problematic, especially if you are very light (under 150lbs) or very heavy (over 280)
My solution was to replace them with Progressive shocks.
I'd add that I wouldn't be too concerned with setting the sag to some specific ideal. The Vulcan is a cruiser, not a sport bike, and although it is a lot "sportier" than other cruisers out there, you're better off setting your suspension for the best comfortable ride rather than optimum handling.
If you're looking to make it the best canyon cutter you can, plan on spending some $$$ for better suspension components...
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