[Vuejs]-How to change select based on another select in vue?

itemsEstacion is an array, that’s why itemsEstacion.estacionID is alway undefined.

I’ve changed your code, move v-on:change="cargarTanques" to first select.

<v-flex xs12 sm8 md6>
    <v-select 
        autocomplete 
        :items="itemsEstacion" 
        item-text="numeroEstacion" 
        item-value="estacionID" 
        prepend-icon="local_gas_station" 
        :loading="loading" 
        label="Seleccionar Destino" 
        v-on:change="cargarTanques"
        v-model="selectEstacion">
    </v-select>
</v-flex>

<v-flex xs12 sm8 md6>
    <v-select 
        autocomplete 
        :items="itemsTanque" 
        v-model="selectTanque" 
        item-text="NumTanque" 
        item-value="ID" 
        prepend-icon="opacity" 
        label="Seleccionar Tanque">
    </v-select>
</v-flex>

cargarTanques() {
  axios
    .get("Tecnico/Tanque/?estacionID=" + this.itemsEstacion.estacionID, {
      headers: {
        Authorization: "Bearer " + localStorage.getItem("token")
      }
    })
    .then(response => {
      console.log(response);
      this.itemsTanque = response.data;
    })
    .catch(error => {
      if (error.response.status != null) {
        switch (error.response.status) {
          case 404:
            this.snackbar = true;
            this.textSnackbar = error.response.data;
            break;
          case 500:
            this.snackbar = true;
            this.textSnackbar = error.response.data;
        }
      }
    });
}