Since you don't stick your vac into the sand as you would when doing a gravel vac, sucking them up shouldn't be an issue. If you stir around the sand with your hand, then you won't bother them. Keep in mind, too, that newborn assassin snails stay beneath the substrate for their first few months.
If you have planted tanks, you don't need to stir up the sand.
75: blood parrots, featherfin cat, emperor tetras, turquoise rainbowfish, BN plecos, japanese trapdoor; 46: WCMM, gold inca snails, ghost, bamboo & amano shrimp, kuhli loaches, rummynose & ember tetras, endlers, platies, flame gourami, guppies; 16: pygmy, peppered, loxozonus corys, otos, assassins, RCS ... ~ Research PRIOR to purchase.... ~