The cable slack will vary a bit depending on how much play there is in your tow bar components. Since the brakes are applied as the toad pushes against the tow bar, that's the condition where you want the cable adjustment to be correct. Don't shorten it so much the brake lights come on when the play is stretched the other way though. On a new installation, some re-adjustment may be needed as the initial cable connections settle in and stretch a bit.
Whether NSA's optional vacuum pump will make any difference or not can be determined with a simple test. With the standard ReadyBrake installation, pump the toad brakes a few times to dump the vacuum, and then drive your coach/toad over a few mile planned route with a variety of braking conditions. Then start the engine on the toad and leave it idling while you run the same route again. If there's a noticeable braking improvement, then the vacuum pump would be helpful. If you don't notice any significant difference, save your money.