We having another issue with the vLookup where it previously prefilled a specific field in the child and it was working fine. Now the parent list has exceeded the 5000 item threshold and it does not prefill the field anymore.
Please see attached image about the message when editing the item.
The field is indexed in the child list. The parent list field is the autogenerated List ID field which is always index as far as I know.
I have found a work around for this issue. As the child list had a Lookup column to the parent list which exceeded the 5000 item limit mark, I created a new column in the child list as set it as text field. Then in the vLookup settings from the parent i set to prefill the new text field with the parent items id. Then I added the new field equals curentitem:id in the query and now it works again.