The customer's Sage Intacct sign-in

Vendor tax IDs, Social Security numbers and bank account and routing numbers never come to the partner application. The customer's own Ledger Doctor sends them to Sage Intacct instead, with a web services sign-in you send it. You never see the values, only whether each one was set.

About 70 seconds, with captions. Sample company, made-up data.

Sending the sign-in

Create the web services user in the customer's own Sage Intacct company, as for your own loads (Sage Intacct setup). It's the customer's account. It will later also send detailed transactions such as GL detail and open documents, so give its role what those need too, not only vendors.
On the customer's page, Connection (for each environment), choose Send this sign-in to the customer's Ledger Doctor. It sends the same sign-in this application uses for that environment: the company ID, the web services user and entity, and the client application. Your XML sender details stay here. It goes encrypted for that customer's install only, through myledgerdoctor.com, which can't read it. It goes only once a paid project covers the company.
A company that's a location or department under another company in a combined chart uses that company's Sage Intacct company ID, web services user and client application (its Connection page says so). It sends that company's sign-in, for this company's vendors, with that company's entity, the one this company's loads go to. The card says whose sign-in it is. Enter that company's connection first. Each company's own Ledger Doctor gets its own copy, and you withdraw each one separately.
The customer configures nothing, and you don't need remote access to their server. Their Ledger Doctor keeps it sealed to their server, hidden from their users. Their Migration page shows only that you sent a sign-in for the company and environment, when, and when it was last used.
Withdraw it deletes it from their server at its next check-in. It's also deleted when either of you ends the connection, after GP's sunset date (GP's last day for the company, on the cutover calendar; go-live alone doesn't delete it), and when one of their Ledger Doctor admins deletes it. After GP's sunset date their Ledger Doctor takes no new sign-in, so Send isn't offered. Send it again any time.
Their Ledger Doctor keeps a sign-in only for Sage Intacct's own address (api.intacct.com).

Sending the vendor fields

On Records > Vendors, under Sensitive vendor details, tick what to send (tax ID, the 1099 form and box, payment details) and choose Send from the customer's server. It lists each included vendor's GP ID and Sage Intacct ID (as you renamed it on Records), nothing else.
Create the vendors first. At its next check-in the customer's Ledger Doctor reads the fields from GP: the tax ID, the 1099 form and box from the vendor's setup (named as GP's 1099 box setup names them), and the vendor's EFT bank on its remit-to address (else its primary address, else its only one). It signs in to Sage Intacct with the sign-in you sent and updates each vendor.
The answer shows on the same card: for each vendor and field, set, not in GP or not sent, and Sage Intacct's error with the numbers taken out. A vendor Sage Intacct or GP doesn't have is named. A routing number that isn't a valid 9-digit US one, a field Sage Intacct's vendor definition doesn't have, and a 1099-MISC box 7 (1099-NEC box 1 since 2020) aren't sent, with why.
When the customer's Ledger Doctor has no sign-in for that environment, the request is refused and says so. Send the sign-in, then ask again.
The checklist step Vendor tax IDs, 1099 setup and payment details (sent from the customer's server) ticks itself off once the latest answer for the environment has no errors.

Sending, withdrawing and asking are an Approver's, and each is in the security log, never with the sign-in.

When the customer allows their data to be downloaded

Without Sage Intacct Web Services, the customer's Ledger Doctor can't set these fields in Sage Intacct. Their admin can tick Allow my partner to download my data for implementation and data migration on their Migration page, for your connection. Then each package also brings the vendors' tax IDs, 1099 setup and EFT bank details, encrypted for your application alone.

Ledger Doctor Partner keeps them encrypted, apart from the rest of the package, and never shows them on a page or writes them to a log.
On Records > Vendors, the vendors import file carries the tax IDs. Import it first. Then download Vendor 1099 setup (Sage Intacct's Vendor 1099 transaction update), Vendor ACH details (Update vendors for ACH) and, for banks outside the US, Vendor bank file payment details, and import them through Company > Setup > Import data. Each download is in the security log, with who and when.
A routing number that fails the US check isn't put in the ACH file: the card names each vendor left out and why.
In the 1099 file, a 1099-MISC box 7 goes in as 1099-NEC box 1 (nonemployee compensation since 2020), and a 1099 box with no type goes in as NEC, the same way the vendor records do. The card names each vendor converted, and the file's to-check list (in a zip with the file) has them by line.
They're deleted when the customer turns the setting off (at their next package), when the connection ends, and after GP's sunset date.