Applying Vaseline on the scalp can be tricky and since you have more chances of damaging your hair than improving it, perhaps you should quit this idea.
As for improving the strands, be it by reducing the frizz or by protecting the split ends the key is: tiny quantities. Don’t cake the hair with petroleum jelly or it won’t be able to absorb it and you’ll end up with a very greasy hair.
Natural tricks to increase hair growth
Unfortunately, Vaseline doesn’t work to promote hair growth but there are other natural solutions that can help out.
The main thing you can do to speed up your hair is to improve your diet to increase the intake of vitamins essential for hair strength and vitality. You can also use supplements if that’s more convenient for you, although consuming them as foods would also bring other benefits. Focus particularly on vitamins A, C, D, E and biotin.
Finally, you can also massage your scalp gently to increase the blood flow in the area, which in turn would mean more oxygen and nutrients brought to the follicles. If you do it using an oil, such as vitamin E, you will also be nourishing the scalp simultaneously.