With advice like EG's I can see why this post has been dormant for over 10 years.
Firstly, vermiculite is just as fire resistant as perlite. They are BOTH exceptional at absorbing water. And unless you're referring to an almighty creator of everything then no-one "designed" vermiculite, it's a naturally occuring mineral. I'm not saying wet vermiculite won't cause a steam explosion (and I'm sorry to hear of your accident) but such an incident can easily be avoided by slowly curing your diy bricks, gradually increasing the temp until finally reaching the firing temp. By that stage all water should have been purged.
Secondly, I'm pretty sure readers would know that if a recipe calls for cement then they would know not to go and add some aggregate for good measure. If the recipe required that the cement be made into concrete it would mention it.
Speaking of recipes why not just keep it simple and make your firebricks out of sand and sodium silicate (an almost 100% silica brick) if all you need is a refractory material. Or add vermiculite (or perlite) if you need insulative qualities as well. To increase durability I coat them with an aluminum oxide based render, but this is definitely not required.
IMHO...
