If it's an issue- which it sounds like it's not anymore(?) -
The vinegar can obviously be watered down. Baking soda- hmmmm... I'm pretty sure you could mix it with RO water to make a ph+ solution and it would retain it's alkalinity. Careful with that stuff in quantity it's very salty. But then the stuff I use is salty too I suppose. For ph+ I use sodium hydroxide (aka lye), which is sold as ph+ for hot tubs. I mix it with water to make my ph+ solution.
In researching the subject I find that people have posted various comments about how it is 'bad' to use lye as ph+, none of them based on any actual evidence as far as I could see.
One argument is that it will introduce toxic salt levels, as it is sodium. (As is your sodium bicarbonate)
But to adjust the ph of a 1000 ppm solution I only need I use about 30 ppm of lye, at most. So I don't think that argument holds water.
I've used it for over five years and haven't noticed any difference between using it and using the grow store ph+
Vinegar has the reputation of not staying stable- that is, it's effect might wear off and ph will start to rise. I haven't tested that though I probably should. I like I said, I used it for many years.
I stumbled across a bottle of ph minus I didn't know I had and have been using that for the last year instead of vinegar.
Your ph pen should always have the probe stored wet or it will start to malfunction.
Hope this blathering is helpful....?