I also have problems getting my stay credits applied to my account. Marriott says I have to go through Vistana to update. Even when I send them my receipts it doesn’t get done. I’m dropping my Bonvoy Brilliant card at renewal.
They could be referring to updating your Bonvoy number in your Vistana account. When SPG migrated over to Bonvoy, despite doing all the alignment required in the SPG and Bonvoy systems, Vistana's system didn't quite pick up the correct information. For about 6 months, despite seeing a Bonvoy number in my Vistana account online, none of my Vistana stays would post (or show up in my Bonvoy "Trips" list). I called Vistana, asked them to delete the number in the system and re-enter my Bonvoy number. Ever since all stays post with credit and all upcoming reservations show up in "My Trips" on my Bonvoy profile.
As to getting credit retroactively, yes, this is a total PITA as post-stay these trips show as a promotional rate, which technically doesn't qualify for Bonvoy credit. Takes a LOT of effort to get someone to understand this was a timeshare stay, timeshare stays are indeed eligible and to then get them to manually post the credit.