Upgraded to 8.0.0 (Oreo) and Notification Vibrations Won't Stop
I experienced the same issue. I ran a google search, and came across this thread. I hope it helps for you too!

https://forums.androidcentral.com/google-pixel-pixel-xl/828108-turning-off-vibration-oreo.html

(One mentioned in that thread, that Twitter doesn't have a setting to shut it off. I don't have Twitter installed, so I can't verify) I did go into all of my notifications (each and every one) and had to turn off the vibrate. It is very tedious, but hopefully helps you too.

UPDATE: Just in case that thread mentioned above, disappears. Here is the quote from the user on how to disable the vibration on each notification

"Super annoying thing Oreo started, happened to me too. For some reason Google thought this was a good idea?!

Go into settings, apps and notifications, app info, messages, app notifications (again sheesh) and then you have to go into each and every category and uncheck vibrate. Thanks Google"