Would it be possible to trigger automatic purchase of a concurrency line in VAPI with API?

Yes

Which endpoint should be utilized for this

Yes, you can try creating a payload with this as the request body:

POST /subscription/your-subscription-id/concurrency
Content-Type: application/json
Authorization: Bearer your-auth-token

{
  "quantity": 5
}

This endpoint will purchase the specified number of additional concurrency lines for your subscription. The endpoint includes proper validation and will return an error if there are issues such as insufficient credits or missing payment details. There's also a corresponding DELETE /subscription/{id}/concurrency endpoint available if you need to remove concurrency lines later.

Where I can find my Subsription ID, it looks like it's not Org ID nor Wallet ID.

You can find your Subscription ID in the Billing or Credits section in your vapi dashboard..

I don't find any subscription ID in the Billing page, apparently my org is on Ad-hoc Infra plan.

If you’re on the Ad-hoc Infra plan, vapi likely doesn’t provide a Subscription ID and you can use your Org ID instead..

Unfortunately using Org ID does not work, I receive Invalid Key error while using Valid VAPI private key.

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