Hydrostatic pressure of a 24" column of glass works out to 2.17 psi.
Although the compressive strength for Vermiculite board is close to 150 PSI & its Modulus of Rupture is 36. This is equivalent to a 2 pound point load breaking a 1" x1" bar over a 12" span for a 3 point bend test. Although it is strong in compression - its low tensile and flexural strength require that casting vessels be engineered in such a way that they cannot flex (or be in tension) when loaded.