I'm not sure if Tasty Puff flavoring is all VG or not. Usually flavoring is PG or alcohol base. You may want to check that just for your own info.
The maximum flavoring is usually 20%. Adding flavoring is not linear; ie. twice as much flavor is not always twice as good. You should try different levels of flavor. I mix 4ml to 20% and then dilute 1ml each to 15,10, and 5% to try the different levels of flavor. You can fine tune from there.
Distilled water contains nothing but water. Spring water or bottled water is not the same. Only distilled water is distilled water. There is nothing in it to go bad, just keep it covered and avoid getting anything in it. The usual maximum for diluting with distilled water is 10%. This should thin the VG enough, although it may not be thin enough for some cartos.
Relax, Enjoy!