[travis]

Vermiculite retains moisture which is why it makes a superior layer in which to germinate the tomato seeds. I have used vermiculite successfully as the sole component of sprouting medium at times. However, I prefer to mix it with peat, coir and other stuff as well.

Spagnum moss (peat moss) also retains moisture, and serves the same purpose, but characteristically clings to the sprout heads, dries out as the sprouts push upward, and sometimes causes "helmet heads" or "stick tights" that inhibit the quick opening of the cotyledons (seed leaves) especially in older, weaker, or dried out tomato seeds.

I do not see perlite and vermiculite as serving the same purpose in a seed starting mix. Yes, they can both "lighten the mix" in terms of gross weight (if that really makes a difference in purely seed starting mix), but perlite is light, fluffy, and dries out quickly, usually floating to the top of the germination cells, and blows around when you expose the tray to natural wind or a fan. Personally, I find perlite a big PITA to deal with when mixing and loading starter mix into trays when working out of doors or where ventiling fans are running.

Again, vermiculite retains moisture, and basically substitutes for a baked clay or a peat product in that regard when formulating a mix. Plus, its basic grain size is greater than peat, and clay is problematic in other regards, and vermiculite at the correct grain size will not generally cling to the sprouts as does peat and wet clay.