I've not bought an unflavored juice, but I did over power a DIY juice with the flavor once. (Mrs. Katz distracted me while I was mixing, I'm highly distract-able (so-called ADD).) I had to save the juice by adding more unflavored liquid which I brought up to the nicotine strength I needed to keep the nicotine the same. That said, I've not seen anyone selling an unflavored
e-liquid without nicotine. (Not saying such a thing does not exist, just that I've not seen any vendor carry it.)
I do keep a 60/40 pg/vg mix with 0mg/ml nicotine for flavor testing purposes (nicotine is expensive, pg and vg are cheap--also all my DIY juices are 60/40 because I prefer that ratio) but I make that myself.
If this is a nicotine issue, I solved a problem I had with juice being too strong but the next lower level of juice (I wasn't DIYing then) too weak to cut the juice with VG or PG. PGA is also an option though I've not tried it. I've also used distilled water before--but I found that the water tended to irritate my throat.