Lars: But as far as I understand it, vi never has the meaning of
"again".

Rene: Yes, I think you are right in the narrow sense.
“Pati” is usually used for “again.” I looked at vi in the several pages
devoted to it in the PTS Dict. and find that because vi can act as an
intensifier (PTSD’d 4th meaning), it sometimes means “re- ...” (or
something close to that) when the word to which it is prefixed denotes
an action:

-- va.t.tati = to turn; viva.t.tati = to re-turn, to start again (a new
world cycle)
-- vatteti = to cause to move; nivatteti = to lead back; viniva.t.teti
[vi + nivatteti] = to repeat
-- vijjhati = to pierce; vinivijjhati [vi + ni + vijjhati] = perforate,
pierce through and through

It seems that sometines the vi- implies a repeating action:
-- pharati = cause to expand (one meaning); vipphurati [vi + pharati] =
vibrate, tremble, quiver
-- ravati = make a noise; viravati = to rattle
-- langhati = to jump over; vilanghati = to jump about; hatthavilanghaka

= jerking of the hand

-- Rene