I think we need to know more about the whole story. The OP does have vermiculite, it was in the first mix. Also in the first mix was coconut coir. It's not clear, to me at least, whether this was mixed and the plants planted into it, or seeds put into it, and then watered, or if it was mixed and hydrated and then the seeds/plants planted when it was damp. Both coir and vermiculite can be pretty dry. While over-watering is a much more common cause of plants dying, being too dry also has to be ruled out.
Were they seeds that germinated in the mix or were they transplants from somewhere else? If they were transplanted when very young, were they handled gently by their seed leaves without touching the stem? If young seedlings were they looking fine and upright and then keeled over from the base (damping off)?
I'm wondering if the "burnt wood" is actually charcoal since it was mentioned with the "red rocks" (put at the bottom of the pot?).
Vermiculite, assuming new, isn't going to be a problem in 48 hours, it would take much longer than that to lose it's structure. You can start seeds in plain vermiculite and grow them in it for some time, initially without fertilizer.
Having said all that, from the follow up "I experimented with this mix and planted some more kai lan (as it grew very well). When i was dumping the soil back into the big soil container i saw some sprouts. I was under the assumption that my mix was killing the plants", I'm not sure if there is still a problem?
To answer this question "A garden supplier said that my mix is too complicated and it might not be balanced. Do you think i should simplify my mix? I want my plants to get as much nutrients and minerals as it can get."
Yes, the second mix is too complicated, I would simplify it. I'd agree with going with a good commercial potting mix (or seed starting mix whichever is appropriate). The danger with aiming for as much nutrients and minerals as possible is in overdoing it. Remember that plants make their own food from light, the nutrients (minerals) are to help with that and other processes. A new seedling has all the nutrients it needs stored in the seed without any needing to be being supplied for the first few weeks.
