On the contrary, I found the less vacuum lines I had open with the BBD, the better it ran with better mileage. Before I swapped it out, I had one advance line running to the distributor. That's it. The charcoal canister went out the window, as did the EGR set up, I have no brake booster so that was capped, and the biggest improvement I found was plugging the PCV port. I know this system isn't the most environmentally friendly, but there's no emissions laws where I live that require any of those vacuum systems. And honestly, these things are so old those emissions systems wouldn't make much difference. I doubt your poor advance, or your lack of other assorted vacuum systems, is causing poor mileage.
What port is your distributor advance hooked up to? And do you have any open ports on the carb? Try capping them first. And check for any sort of vacuum leak below the butterflies, like the gasket between the spacer and the manifold
->'79 CJ7 (VIN Says it's a 5, wheelbase says otherwise), 6 inches of spring lift, awaiting an MC2100 on a teamrushed MSD'd 258
-->350 time. '95 block SBC, stroked to 383, dremeled heads and big 'ol intake, maybe a Holley 750 CFM, not sure yet
--->"Safety? Buy a Volvo. Never buy a CJ, or a Chrysler Jeep knock off, for safety, fuel mileage, reliability, handling, ride, comfort, aerodynamics, quietness, convience, stability, low maintenance cost, performance, or ease of use. The only reason to buy a CJ is because you are absolutely, incurably nuts." -OldTexas