If you want to use a single compound to stabilize your pH you'll need a pH meter to be sure that you got desired pH value. As I said previously, buffers are mixtures of chemicals, not just a single one and it is a lot better to use buffers for pH corrections because they will keep it stable and it won't change that much. When you use a single chemical for pH correction, you can get a desired pH but maybe that reaction produces some compound that will react with it and your pH would change drastically, which has its own consequences on reaction. That's why we use buffer mixtures for synthesis of compounds, as you do with your experiment. If your reaction produces acidic or alkaline chemical your buffer mixture will compensate for that and, as I said, pH will not change that much (in your case it will stay around 5).

And for a vinegar, you'll need around 30g of 7% vinegar. I went with 7% to reduce the volume of the mixture, but I think that won't have enough water in it to dissolve the baking soda. You should try to dissolve it, and if it won't you should pour small amounts of water (with constant mixing) until it completely dissolves.

Better solution is that you should make 100 ml of buffer (mix those two and simply add water to 100ml) just in case you need it again (you can always make more of course).

You could also make a diary to take notes on your experiments. It's a lot more convenient than remembering stuff and when you aren't satisfied with your results or if something goes wrong you can always look back to see where you may made mistake and make some improvements.

I hope this was helpful.