I use the vermiculite since several year in all the acid substrate plants , with no problem , it is a very good 'allegeant'
for me this one is more interessant than perlite with the blond peat.
inside le perlite: SiO2 ; AlO3 ; FeO3; MgO ;Na2O ; K2O ; CaO
vermiculite chimic formula : (H,Na,Ca1/2) (Mg Al2)Si4O10 (OH)2
have you try to heliamphora : granite or sandstone very presence up there on the tepui ?