Have you added the darts to the item list where weapons would go?
Also, once you do this, go under the monster tab and force the darts/spikes as the default.
Also, with ranged weapons, don't you need to add the materials to the consumables tab so that vtank can make more? I have never used a ranged weapon with vtank, so not sure about this.
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In-Game as Bluballz or Vash