Killing lice with Vaseline might sound like one of those old wives' tales, and perhaps it is, but those old wives knew what they were doing. Vaseline is a perfect substance for smothering and killing lice because it is thick enough to stay put on the scalp and not allow any air through that could sustain the head lice. Many other remedies, including thin oils, can drip off the scalp and leave the lice perfectly healthy.
To kill lice with Vaseline, the entire scalp must be coated in it. Any hair that doesn't get coated by the Vaseline doesn't need to be coated as long as the entire scalp is covered. Lice don't live on the hair shafts themselves- they hook themselves into the scalp. As long as the scalp is covered, you can kill lice with petroleum jelly of any brand. The secret is really in the suffocation process. Killing head lice must be done by either removing them from the scalp or cutting them off from air for eight hours or longer. If the Vaseline is left on the lice for eight hours or more, it will kill lice effectively without any poisons needed.
The real problem with using Vaseline to kill lice is that it is extremely difficult to get it out again. The very properties that make it an effective weapon against lice make it stay on the hair so well that it can take days to get it out. When you are ready to get it out, the best remedy is to use a grease-cutting dish washing liquid on the hair. Dawn is a great product that is concentrated enough to cut through Vaseline grease and loosen it in the hair.
For your best chance of getting the Vaseline out again, wash the hair two to three times with Dawn or a similar generic dish washing liquid. After that, wash it a few times with a strong shampoo. A conditioner is not needed in most cases. Even this much washing may not completely get it out, though. It may take two or more showers, using this method each time, to get the Vaseline out of the hair.
On the bright side, the lice will be dead. Unfortunately, the eggs won't be. To kill lice completely, you then have to comb through the hair again and again with a lice comb to remove all of the eggs possible.