"Just wanted to share an Example of what V.A.N.S. looks like, especially for new players to Fallout 4, or ones that haven't used it before and are wondering how it works.

(Example was recorded aboard The Prydwen)

Briefly summarized, V.A.N.S. is a Perk of Intelligence (requiring only 1 INT to Unlock) that lets the V.A.T.S. interface 'show you the way' to your marked objective/quest/mission location.

To use V.A.N.S., simply hold the Hotkey for opening VATS for a short time (eg. on PC, instead of hitting 'Q' quickly once, to enter V.A.T.S., hold it down) - let go of the 'Q' key, after about two full seconds or so - even if the Pip-Boy has not fully opened yet. Then hit 'TAB' to close the Pip-Boy right away, once the 'green trail' is seen.
You do not need to move or turn at all, until after closing the Pip-Boy fully (for example, I do not use the mouse at all to use V.A.N.S., on PC, I just open the Pip-Boy with 'Q' and close the Pip-Boy with 'TAB').

(Perhaps this will help those having issues with opening/closing/viewing the 'trail'...)

If you've played Skyrim, it is similar to the Clairvoyance Spell in that game, as it will then illuminate a 'trail' that looks like a tube of smoke along the ground, that you can follow (but in Fallout 4 it is Pip-Boy Green, as opposed to Blue).

While this doesn't seem needed as Fallout 4 has both Map Marks (on the Map in the Pip-Boy and at the bottom of the Screen in a Compass) and Floating Markers (that can be seen on Doors, etc). V.A.N.S. is a helpful in a slightly different way. It shows you the 'Main Path to take' to get to your Marked Objective/Quest/Mission (as in, "go to this Street and use this Road").

For example, if you are jumping around in the hills or lost, it will guide you out of where you are with the 'green steam trail' and show you how to get back to the main roads, then it will illuminate which way to go from there, too.

The Map Markers that are normally available only show the overall/general Heading, like a Compass (N, E, W, S) and not 'how to go around this thing in front of you' or 'how to get back to the main road first'. That last part is where the usefulness in V.A.N.S. shines; in how it can guide you from exactly where you are, first to the nearest main path, then how to go to your next objective.

Try it out on your next Character perhaps - and have fun in Fallout 4!"