How would Laura know about Josette? If Barnabas was supposed to be just a boy how could Josette be there? Barnabas met Josette in Martinique and she came to Collinsport for her wedding. Angelique would have been there, too. I am completely confused and I have seen this part of the story before.
The idea of Laura in 1966 knowing who Josette is doesn't really make much sense. Neither the Laura who died in 1766 nor the Laura who died in 1866 would have lived during the same time as Josette, who was said to have died in 1822 prior to the arrival of Barnabas and the subsequent history revision.
The ghost of Josette, however, is originally presented as a sort of "Casper the Friendly Ghost" character prior to the introduction of Barnabas, and serves primarily as a protector to David. It's possible that the Phoenix is simply aware of her presense and her determination to protect David from her rather than having actually "known" her in life. It's also possible the 1966 Laura (pre-Phoenix) had a few run-ins with this spirit, and the Phoenix retains all of Laura's mortal memories. Don't know where she got Josette's necklace from; this would seem to imply she DID know Josette in life, but it's also possible Josette arranged for her to have this necklace or altered her portrait to include it just so Vicki would recognize it and realize something wasn't quite right about Laura. Josette's role in the Phoenix storyline was helping Victoria discover Laura's secret so she could protect David.