The NOW is food grade, most likely. If it is USP it will state that, clearly. Although some people like it, I stick with USP grade, which has a guaranteed level of purity. (Food grade may not be bad, just that with USP there is more certainty).
In the bandaid section of a CVS or RiteAid there should be Glycerin USP skin protectant. It is a clear, short, 6 ounce bottle. The only ingredient will be glycerin 99.5%, possibly followed by the word "anhydrous". It costs about $5.
If you already have eliquids, then you would only be adding VG to thicken it slightly and make more vapor. i would add just a handful of %, then shake the bottle and test it. Any VG you add will cut the nicotine level and flavor. If you start with 24 mg/ml eliquid, add 10% VG (3 ml added to a 30ml bottle), then the result will be 22 mg/ml.
Ask at the pharmacy desk for an oral syringe. A lot of places will give out one or two free, otherwise they have them in the child care section, cheap. This makes it simpler to add a controlled amount of VG.