In practise we can't use persistent messages for this, but you can fetch the latest messages that correspond to your subscription from a REST API that caches the messages: https://api.digitransit.fi/realtime/vehicle-positions/v1/hfp...

(The reason is that we use MQTT topics creatively: instead of one topic per vehicle, the topic changes dynamically to provide more filtering criteria such as the route and the next stop of the vehicle. Thus, most of the per-topic persistent messages would be obsolete.)