We also use Vermiculite for incubation as it holds the moisture really well and we successfully hatched royal's and coastal's with a 100% hatch rate too! So will be sticking to tried method's for this year also.
As for substrate in the viv for laying we left the snake's with their normal substrate and added nothing extra for the egg layer's and they were fine but we did notice that the female boa hung on till we placed a tub in for her to give birth in but this was again just a tub with no substrate in it or anything.
Oh and the boa' will be going together very soon so watch this space!!!!! :) :) :)