Your plant looks very healthy to me -- I've had branches occasionally fall off of mine, and then the plant continued growing happily without any further problem. Using the cactus/succulent potting mix is certainly a good idea (I don't think I'd add the vermiculite to it though, as that can actually retain too much moisture). You want to have really good drainage so that the roots are never sitting in water; the pot should have drainage holes, and the potting mix should pretty much dry out between waterings.
