They dont come fully charged, but about 3/4 charged. You can def vape some for alittle, but you should give them a good charge when you get them.
Like others said, the batteries dont have a memory and if you can throw them back on the charger when they have about 20-25% left, they will last longer that way.
As far as charging them, I def wouldnt charge them overnight and you should try to not leave them on the charger for extended periods of time after the light turns green.
Most chargers have a chip in them that will stop charging the battery or switch to a trickle charge once the batteries fully charged. It is rare, but that chip can fail, and when that does happen, the charger will not stop charging, and continue to charge the battery after its already fully charged. Like I said, it is uncommon, but it does happen. Most of the stories you hear about batteries exploding, catching fire, etc are because the charger malfunctioned and didnt stop charging the battery once it was fully charged. So thats why I wouldnt charge them overnight and/or leave them on the charger for extended periods of time after the light turns green, more or less just to be safe. Better safe than sorry