If the vivarium is going to have a water feature that share with water in dead space it will benefit from having an ammonia/nitrites reducing bacteria population in dead space area. My first choice in such a case would be to use Seachem's Pond Matrix since it's lighter than LECA.

If no such water feature is planned there is no benefit to having anything in dead space since it's there to prevent substrate being water logged until water is drained out and that water should not contact your soil. Good luck !