I'm Korean and living in Seoul.
It is too late to help you this time but I hope this would be help to anyone.
I would like to post the full timetable of
the ferry but this page is too small.
Then I can show you the breif schedule.
You can take the ferry from vladivostock to Sockcho on every second saturday at 10:30am.
It arrived on next day.
Although I'm not a motorbiker, I'm interested in motorbike and dream about motorbike travel around the world.
If anyone has concern I will give little help. Good luck.