The simple info on open tunings is that the three main ones have the tonic note (and the 3d & 5th) at different places.

Open D (DADF#AD) has the tonic ("1") on the first string, the 5 on 2, the 3 on 3, the 1 on 4, 4 on 5 and 1 on six. From bottom to top, that's 1515351

Open G (DGDGBD) is: 515135, so the 1 is the 3d string (or the 5th fret of the 1 string).

Open C, mentioned above, is CGCGCE, or 151513, so the top string is a 3, and the tonic is on 2.

This means that you can't just change to open C to play in C instead of D, because all the notes you're used to will fall other places. In D, that strong 1 on the first string lets you play whole tunes on that one string (John Henry starts at the 12th fret and slippy-slides down to the open 1st string, for instance). In C, you'll have to hit the 2d string to play that same tune, and the 1st string -- the 3 note -- will not be a suitable chord note most of the way through the song.

Each tuning has its own rationale. That 3d on string 1 in C tuning is annoying enough that very few play in open C (compared to G or D). However, each one of them has something in common:

Open = the I chord
5th = the IV chord
7th = the V chord

So when you're playing John Henry, and you get to the end of the second phrase, where it goes into the V chord, you'll want the slide (or barre, if you're playing slack-key/finger style) to be on the 7th fret.

Other commonly-used frets are:
2nd = II chord
10th = bVII ("modal" or "blues" chord, depending on your terminolgy)

So if you're playing slide, you'll need the 2d, 5th, 7th, 10th, and 12th frets, whichever tuning you're in. Dobro players sometimes angle the slide to get other forms of the chords, but that's another story entirely.

And "Vastapol" is from the "Seige of Sevastopol," a 19th C. guitar show-off piece, a multi-part composition complete with cannons and battle sounds. The tuning is just about all that remains -- any tune or other remnant has long since fallen out of the folk memory.

Bob Clayton