I'm in the same boat. I submitted my application online and only received an acknowledgment number, like OP. Now I've received an email inviting me for vetting, but I don't have the person ID or application reference number in the format the online form requires. Sent email to citizenship office no ...

Application Based On: NON-EU - 6 years
Applied Date: 08/07/2021
Application received Date: 10/07/2021
Acknowledgement Date: 10/11/2021 (passport returned+ receipt+ acknowledgement)
E-Vetting: Pending, no update