I know that this is kind of late, but this is what I found works, after months of testing. I like to do this on server, and multicast, that way no one sees them lag, but I’ll try and figure out a more optimized way to do it. When when you enter the vehicle, you need to call a run on server event. This even should pass through your player, the actor you want to attach to, and your socket name. Then you want to call that player and get the character movement, set movement mode to none, and the most important part. You need to set Ignore Client Movement Error Checks And Correction to true, and then attach to actor using all the stuff you passed through that event. Just be sure to reverse all of these when exiting the vehicle. Ignore client error movement checks will allow you player to stay where he should without the server correcting him. This is the most critical part of not lagging. I also call a multicast that just disables the collision on the mesh and the capsule. I hope that this helps you, and if it didn’t I’m sorry, but hopefully it will help someone else in the future.
There is a link to a visual pastebin that you can copy.