What is the main difference between dispersal and vicariance?
one leads to allopatric speciation, whereas the other leads to sympatric speciation.
one involves the movement of the organism, and the other involves a change in the environment.
one depends on a genetic mutation occurring, and the other does not.
one involves closely related organisms, and the other involves only individuals of the same species.

One involves the movement of the organism, and the other involves a change in the environment.

Explanation:

Most phylogenetically related groups have distribution patterns resulting from dispersion or vicariance. In the first of these processes, the most recent common ancestor of a given group of organisms originally occurred in only one of today's occupied areas, then dispersed to other areas - overcoming pre-existing barriers - in which descendants survived. In the occupied areas, differentiation processes may occur resulting in the formation of new species.  In vicariance events, the ancestral population occupied, to some extent, the sum of the areas currently inhabited by their descendants, and was divided into smaller populations by the emergence of barriers that caused isolation between subpopulations. The appearance of the barrier causes the disjunction (separation) observed and corresponds to an event that affects all or most of the biota.

From this we can conclude that the main difference between dispersion and vicariance is that one involves the movement of the organism and the other involves a change in the environment.