I think there have been questions about it in the past. When I searched, I found that mainly the concern with PEG400 might be some sensitivity to it, similar to PG. I have some juices with PEG400 steeping but I did vape some for a few days and I don't appear to be "sensitive" to it... so far.
I couldn't determine one way or another that it was totally unsafe, but the concerns raised either seemed a bit alarmist, or to be surrounding the possibility of sensitivity to it, really.
I would probably say do your own search if you want to use it (and for as long as you need to search). I'm comfortable trying it based on what I found, but that doesn't mean you will be. It's certainly your body after all so I wouldn't recommend it to anyone without the caveat that they should look for themselves, etc.
Anna