Actually the distinction has some intermediates.  Your book makes the point that the difference is primarily when the barrier occurred in relation to the movement.

Sometimes migration of a species can happen rather quickly.  Many weedy species exemplify this.  This fairly rapid range change is called Diffusion.  A slower process that involves slow migration with adaptation (and sometimes speciation) along the way is called Secular Migration.  While diffusion may take a few years, secular migration may take thousands.
 

Dispersal--Diffusion---Secular Migration------à Vicariance
Existing Barrier ----------------------------------------à Forming Barrier
"Active" --------------------------------------------------à Passive
Fast --------------------------------------------------------à Slow

Now think about how each of these processes works with extinction.

When you see a disjunction it can be due to long distance dispersal, diffusion or secular migration with extinction, or to vicariance.

The Role of Chance

Long distance dispersal is common in some groups (e.g. some ferns) and very rare in others.

- Vagility, Pagility and Phoresy -

Chance is involved.  What effects chance? Number of propagules, number of opportunities for dispersal (flowerings, years etc.).

For example, maybe there is only a one in a million chance of a seed dispersing from a given species to a new area in any given year.  Over 3 million years the odds start to look pretty good.

What vector is responsible for most successful dispersal events right now?