@Luca Perico , Please try these CLI commands to delete the network profile,
NETWORK_PROFILE_ID=$(az network profile list --resource-group yourResourceGroup --query [0].id --output tsv)
az network profile delete --id $NETWORK_PROFILE_ID -y
After deleting this profile, you can then delete your VNET. Let us know if the above solution doesn't work in your case for further analysis.
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