Here is an updated graphic showing how to quickly complete tasks. As @Magnusimus states vault keys are a hard to come by resource so be sure you really want to spend it for this task. If you have the time to spare and would be doing explore anyway (perhaps for traitstone farming) then maybe see if you encounter treasure gnomes this way to complete the task. Whatever you find fun and suits your play style. Anyhow, here’s the graphic. Feel free to share with your guilds/alliances