Is there a way the Route Optimization API can work out which drops should go on which vehicle? Just I added the vehicles but all drops have gone on one vehicle despite two existing vehicles.
By default, it minimizes transport times. If this can be done with one vehicle, it is most often superior to a solution with two vehicles (since for example you save at least one return trip to the depot). However, if you introduce constraints such size and capacity, time windows or operation time of each vehicle, a solution with one vehicle might not be feasible anymore. Vehicle specific constraints are described here whereas service/shipment specific constraints are described here. Another option is to change the objective function from "min transport time" to "min-max transport time". This minimize the maximum of each vehicle`s transport time and you will probably end up having two vehicles deployed. However, this usually comes with higher overall transport times. It is described here.