Hi there, so sorry for the delay! I read this on my phone while I was out and then completely forgot about it by the time I was back at my computer to type out a proper response...
Unfortunately, these interactions around the necklace are out of sync between the two routes in a weird way. I never figured out a clean writing solution since Vasily's last weekend event is so short, but here is a rundown to get all the content (leads to Ogata's full ending and a lesser ending for Vasily):
[Spoilers for the game below]------------------------------------------------
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- Do NOT start Vasily's route during Weekend 1
- Start Ogata's route normally during Weekend 1
- Do at least one cat sort of thing (they pop up during Usami's and Ogata's routes)
- Do Ogata's dentist event during Weekend 2 in the morning BEFORE starting Vasily's route, then start Vasily's route afterward
- Continue both routes normally afterward
- Choose "Ask about the necklace he is wearing" during Vasily's dentist event (available during Weekend 3)
- Choose "Ask about Vasily's necklaces" during Ogata's Weekend 3 event with high romance points
- Choose "Tell him about the necklaces" during Vasily's park event (available during Weekend 4 before the party)
This will provide you with all of the content related to the necklaces during the game, but it will mean that you won't be on Vasily's final part to get his full ending due to the way the timing works out... It will mean that you get extra friendship points between Ogata and Vasily, though, if that is something you are looking for to see how it changes their interactions during the party
I hope this helps, and I apologize that this timing issue is one of those things I never quite figured out how to resolve before releasing the game. Thank you for your patience and kind words! I am sincerely overjoyed every time someone is working to find all the little plot threads and Easter eggs in the game!!! :D