Besides drowning roots, other things can almost kill a plant in 48 hours: salt and pH.
Salt
The coir might have been salty. You can taste it to see.
Many water supplies are slightly salty ("brackish"). If so, can you collect enough rain water to water your plants?
Adding fertilizer or any soluble mineral adds salt. If watering does not make water flow out the bottom drain holes, those slats can accumulate. But usually that takes time to build up enough to kill the plant!
Nitrogen fertilizer is tricky - a little too much is toxic, so avoid over-fertilizing at all costs!
pH
That vinegar test is very rough. If the water or one of your ingredients is too acidic or too basic, you might have a serious problem with pH. Sometimes a drug store will sell pH paper that lets you tell the difference between pH 6, 7 and 8.
P.S. If a plant dies in a pot, don't mix that soil back into your big bag of soil mix. Even if it doesn't have toxic levels of salt, bad pH, or water-holding root-rotting peat, it had dead and dieing roots in it, plus any plant disease that gets started when a plant is weakened or dieing. Throw it away, or spread it on the ground somewhere it will be diluted by healthy soil and surrounded by healthy roots.
If the cost of good commercial potting soil is a major factor like it is for me, find something cheap but suitable that you can mix with cheaper commercial potting soil. For me, it's ground, screened pine bark. Many people use Perlite. Coarse grit is good (crushed granite, "#2 Chicken Grit" is great).
You might try a small bag of more expensive potting mix just so you can see how it drains and feel how damp but airy it stays. Then you'll know what you are aiming for.
It's much easier to learn gardening tricks if you can see and feel them! If some neighbor has a good garden or healthy potted plants, look at the soil there. See if it's drier and airier than yours. Ask that gardener where he or she gets her soil mix ingredients.
