Which has higher priority according to Cahn-Ingold-Prelog priority rules, vinyl or secondary butyl?
The assignment of CIP priorities has some intricate details when it comes to multiple bonds.

First, the general procedure (Wikipedia):


  
  If there is a tie, we must consider the atoms at distance 2 from the stereocenter — as a list is made for each group of the atoms bonded
  to the one directly attached to the stereocenter. Each list is
  arranged in order of decreasing atomic number. Then the lists are
  compared atom by atom; at the earliest difference, the group
  containing the atom of higher atomic number receives higher priority.
  If there is still a tie, each atom in each of the two lists is replaced with a sub-list of the other atoms bonded to it (at distance
  3 from the stereocenter), the sub-lists are arranged in decreasing
  order of atomic number, and the entire structure is again compared
  atom by atom. This process is repeated, each time with atoms one bond
  farther from the stereocenter, until the tie is broken.
  


Now, concerning double bonds in particular:


  If an atom A is double-bonded to an atom B, A is treated as being
  singly bonded to two atoms: B and a ghost atom that has the same
  atomic number as B but is not attached to anything except A. In turn,
  when B is replaced with a list of attached atoms, A itself is excluded
  in accordance with the general principle of not doubling back along a
  bond that has just been followed, but a ghost atom for A is included
  so that the double bond is properly represented from both ends.


I have constructed a table of these atom lists for the vinyl, isopropyl and sec-butyl rests:
$$\small
\begin{array}{llll}
\hline
\text{Dist} &amp; \text{Vinyl} &amp; \text{Isopropyl} &amp; sec\text{-Butyl} \\
\hline
1 &amp; \text{C} &amp; \text{C} &amp; \text{C} \\
2 &amp; \text{C} &amp; \text{C} &amp; \text{C} \\
  &amp; \text{(C)} &amp; \text{C} &amp; \text{C} \\
  &amp; \text{H} &amp; \text{H} &amp; \text{H} \\
3 &amp; \text{(C)} &amp; \text{H} &amp; \text{C} \\
  &amp; \text{H} &amp; \text{H} &amp; \text{H} \\
  &amp; \text{H} &amp; \text{H} &amp; \text{H} \\
  &amp; &amp; \text{H} &amp; \text{H} \\
  &amp; &amp; \vdots &amp; \vdots \\
\hline
\end{array}
$$

The sec-butyl rest has thus the highest priority of the three, followed by vinyl and then isopropyl.

The ghost atom at distance three is the ghost carbon at distance one, as seen from the ghost carbon that was doubly bonded to it. If the distance 4 was to be considered, no more ghost atoms would appear, as the ghost atom at position 1 does not have any more ghost atoms bonded to it (atom 1 however is bonded to the ghost atom at position 2).