For people with sensitive ears (aka metal allergies), coating your earring posts in Vaseline (or Aquaphor or Neosporin) won't prevent your earrings from bothering your sensitive ears. Here's why:
Vaseline, Aquaphor, Neosporin - these are all petroleum based products that are meant to be a moisturizer or lubricant. These products will work well for helping to slide your earring posts into your piercing when it has been a while since you have worn earrings, but they won't stop you from reacting to cheap, reactive metals.
The theory behind putting Vaseline on your earring posts is that the Vaseline will act as a barrier between your earring posts and your skin. The ultimate goal is obviously to prevent your sensitive ears from having a painful reaction. The problem is that the reaction is caused by a contact allergy to the reactive metals commonly used in fashion earrings. Vaseline, Aquaphor, and Neosporin will all be absorbed by your skin and/or rub off of the earring posts. When that happens, your sensitive ear piercings will be exposed to the reactive metals in the earring posts. Once you have developed a contact metal allergy, even brief moments of contact with a reactive metal can cause a painful reaction. So, while the Vaseline may create a barrier between your earring post and your skin for a few minutes and therefore delay the reaction, it will soon wear away leaving your skin in contact with the allergen once again.
Instead of trying "hacks" like this, and like putting clear nail polish on your posts, the solution is to wear earrings that are safe for sensitive ears. Titanium and niobium are two of the few metals that are safe for sensitive ears (discussed more in depth here). Do your research and choose wisely when making your earring purchases -- you should treat your ear piercings just like you would any other medical implant.