How can simulated annealing be related to the vehicle routing problem?
Simulated annealing is an optimization paradigm, in which (very roughly speaking) you start with some solution, and then try to make local changes that improve it, sometimes making a local change even if it doesn't improve.

Vehicle routing is an optimization problem. It is likely that the simulated annealing paradigm can be applied to it, probably in many different ways (corresponding to different notions of local change).