Here is what I wrote about 'vital' (comparing it to 'important'): "Something important has high significance, but something vital is absolutely necessary. Ex. Your arms and legs are very important, but your heart and brain are vital." http://www.italki.com/question/197328 All three of the words you mentioned have the same meaning. 'Essential' doesn't sound as strong as the other two because it's used far more often, but that's it.

They're very similar. It's just a matter of degree and timing. I think examples will explain best. Important. If you want to get a good job, it is essential that you get a good education first. When you go on a long hike, it is essential to bring plenty of water. Extremely important/time critical. If you are bitten by a rattlesnake, it is vital/crucial that you get to a hospital immediately.

They're all pretty much the same.