In my experience, slowing the cooling using a crockpot on high filled with vermiculite truly DOES allow you to make things with the soft glasses that would crack otherwise.
For example, Bullseye (COE 90) and Moretti/Effetre (COE 104) both respond favorably to slower cooling using vermiculite in a hot crockpot. The slower cooling prevents some of the stress cracking that would occur with some shapes, some thicknesses, and even some of the "touchy" colors. I really don't know if it's helpful for boro, because I am speaking from my own experience and haven't ever tried using it for boro.
Remember, the slower cooling only allows a tiny bit less stress to fix in the glass. The most stress accumulates in the glass as you are working it, and that is why flame annealing helps to release the stress a lot -- but attempting to release residual stress is why kiln annealing afterwards is basically an accepted necessity among beadmakers.
P.S. To avoid any distortions from plunging your item into the vermiculite, dig a small hole with a spoon, place your object inside the hole, and cover it with vermiculite.
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--Terri
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