Yes, I expect white vinegar "NOT" to be able to keep a stable pH at all, and with the consent adding of it to continually lower your pH, I can easily see having other problems as well if using it for any length of time. I have never used it as a pH adjuster myself, but know it wont be stable.
The only thing I use white vinegar for in hydroponics is to soak
grow rocks in after the grow is over. That's to clean the mineral salts off (but only after the plants were done). That cleans them off quite well, and get's them ready for reuse later. But you do need to rinse them out real well after (I often use boiling water), until you cant smell the vinegar anymore, and/or let them sit for a few weeks to let the vinegar degrade before reusing them. But I usually start with soaking in white vinegar to clean the salts off, rinse a couple of times. Then soak in bleach-water to sterilize them too, then rinse a couple of times more before letting them dry, and storing them.
P.S.
I thought you said you were using GH pH adjusters?
10 plants, and about 35 gallons of
nutrient solution, comes out to 3.5 gallons of nutrient solution per plant. Depending on heat and humidity levels, I can see 6 foot tall plants drinking at least 1 gallon each per day (more than 25%). But if their bushy plants, in warm climate with relatively low humidity, and/or have a lot of fruit on them, I can see them drinking as much as 2 gallons of water a day (more than 50%). Water to plant ratio may be a problem, causing too much nutrient fluctuations.
Here is an example of water fluctuations causing inaccurate readings. If you add water in the morning before the first feeding, then take readings and adjust pH to 5.5, and have a ppm of 900. Then through the day the plant is continually drinking water. You take readings later in the day (without the water level being the same as it was with the first reading), you will naturally have a much higher ppm, and pH changes as well. That's because you also have a much lower water level (concentrating the
nutrients in it), simply because the plants have used (drank) about 10 gallons of water. So instead of 35 gallons of water in the reservoir, your down to 25 gallons.
If you were to take 25 gallons of water and add nutrients to it (like mixing a fresh batch of nutrient solution), and take ppm and pH reading. Then you add 10 more gallons of water you will get completely different readings every time (because you diluted it). This works in revers too. If you take 10 gallons of water out, your concentrating everything in the water. So you naturally get higher ppm readings. Concentrating (and diluting) the mineral salts can change pH. Then throw in the added issue of altering pH by adding water that wasn't pH adjusted first. That all adds up to inaccurate readings because there isn't any consistency when/how they were taken, so they cant really be compared with any accuracy.
P.S.
I don't know if the example apply's to how you've been taking readings or not (because I'm still unclear about that), but I used it as an example of how the water level will affect your readings. As well as the importance of taking the readings with the same water level each time, before assuming pH, EC/TDS/PPM readings are "abnormal."