I would not replace the carb, as it is most likely sound. If anything it is a carb gasket, not the carb.
I have had a '68 small block since 1971, and have had many vacuum problems in the past. The C3 has an awesome amount of vacuum hoses all over the car, running the distributor, headlights, wiper doors, some A/C components and more. Seems like every damn thing on the C3 used vacuum for something. Be absolutely sure you have blocked off all these circuits from the engine and re-run your tests.
If the vacuum comes up, start re-attaching each circuit until it drops.
If that does not produce the source of the problem, low vacuum readings may also indicate intake valve stem wear, intake manifold or carburetor gasket leaks including the gasket between the carburetor and the intake manifold. Try spraying carburetor cleaner around these areas while the engine is idling to test for leaks.
Varying vacuum readings at low engine speeds may indicate a problem with worn valve stems, valve guides, or valve seals. Higher engine speeds tend to temporarily cancel out this problem because the engine is producing more vacuum than is lost to these leaking components.
Since you have ben chasing this for some time, how long has the low vacuum condition been around. Did it suddenly just drop from 18" to 10" or has it dropped slowly over a longer period of time?
If it dropped over a long period of time, You now may suspect valve guides or sticking intake valves. Have you run a compression test? That may possibly indicate one of these conditions.
If the compression is low, it is either due to the rings or the valves. Get an oil gun and spray a few ounces of oil into the spark plug hole and immediately re-run the compression test on that cylinder. The oil will seal bad compression rings and the compression will come up. If the compression does not come up, the rings were not the problem and that indicates the valves or valves guides are the problem.
Many years ago, when I was in the Navy, my '68 350/370 hp LT-1 would sit for long times (up to 6 months) while I was at sea, and would run like crap when I came home. I have, on occasion, used Mystery Oil to really clean sticking valves before with great success. Get the engine to normal temperature and very slowly pour small amounts of the Mystery Oil into the primaries of the carburetor. Make sure you are outside when you do this, as it will smoke like hell!
You may need to change the plugs after this.
I have never experienced this, as I have always run headers, but if you have stock exhaust, I have read that a stuck heat riser valve or otherwise blocked exhaust can also cause a loss of intake vacuum.
Best of luck, hope you find a bad hose or a bad gasket somewhere.