Just to chip in my little bit, when my wife was pregnant and the midwife did her home visit, she did the usual questions about smoking etc before the co2 tests.
I said I vape, whilst opening some new juice, and she asked if I would do the same test, just out of interest as she'd never met a vaper in that situation.
My wife did the test, "normal non smoker" result. I did the test, almost as good as a non smoker result, only slightly elevated levels but still well within the usual "environmental" nominal levels.
So as far as I can see, it wouldn't pose any issues for breastfeeding. Only if there was juice directly going into the baby's mouth.