I'd used vermiculite for years with no problem but about 10 years ago i lost a few to being stranded on their backs, they must have been struggling to right themselves all night (as i seemed to find them in the morning) they were in a bad state and died later (from hours to days, i even found one dead on its back) those that died later seemed to have trouble with their equilibrium.
The reason for this happening imo (if they do fall awkwardly, onto their backs) is that as they are reaching about with their legs to grab some thing solid to right themselves with but only managing to grab/grip one small piece of vermiculite per foot, this is obviously much less than the mantis in weight and they didnt manage to right themselves staying stuck on there backs.
It could have happened to others and they did manage to right themselves but i only noticed the effected mantids.
Theoretically this could happen with any loose, granular substrate and its the reason why i use tissue now.
It might never effect you or your mantids but as with me, when it did, quite often it was when i was limited in numbers or sexes of a species or with an expensive or hard to obtain species, more than very annoying.
You could still use vermiculite but it might be wise to place a measured amount in the toe of an old stocking or similar, knot the end and press it into the base of the mantids enclosure, easily gripped should your mantid fall awkwardly on it and it might also limit burrowing prey types from dissapearing into it. This is too much faffing around for me so i use tissue, if i need more humidity i use more depth of it