To answer the actual original question…
Assuming a yoyo is perfect to start with, what can cause vibe later on?
Here is a quick list of things that can cause vibe in a yo-yo. I’m going to assume that by “vibe” we mean -any- visible ‘ghosting’ when the yoyo is thrown, tapped with a fingernail to settle it, and held against a contrasting background.
1) Design/Manufacturing problems.
This is a broad one, so I’ll try to explain as clearly as I can.
This is a case where either due to an unstable design, poor material quality, or deficiencies in the manufacturing and quality control process, you end up with halves that aren’t actually perfect. They may in fact ‘appear’ perfect, in that when precisely aligned with the right matching half, they spin perfectly.
The most famous case of this is old Anti-Yo products. If you “tuned” them, they usually could be made smooth. This was done by changing the alignment of the halves to one another, usually by adjusting the placement of the axle.
Simply disassembling a yo-yo like this is likely to cause a problem… it may not go back ‘exactly’ as it was before, and as such may show vibration.
The other possible issue here is the threads on the half/axle. Some yo-yos are tapped such that the end condition of the hole isn’t consistent. Because you need to have some small amount of ‘play’ between the threads and the axle so that it doesn’t seize up, the axle can be forced slightly out of alignment when tightened. If the end of the hole isn’t clean/precise it can jam against the axle and torque it out of alignment within the threads.
2) Damage to a bearing seat
This is less common than a lot of people seem to think, but it certainly happens. Very tight bearing seats are more susceptible to this, but removing/replacing the bearing lots of times can wear the anodizing on the bearing seat. That allows for the bearing to be ‘loose’ and possibly out of alignment with the halves.
Note: A bearing seat being “bent” is -extremely- rare. I’ve seen many claims of it, 99% of which aren’t actually the case. It would require a huge amount of force in an unnatural direction, or a very weak design.
3) Bad bearing
Debris, poor quality, etc. can cause a vibration from the bearing itself. This usually doesn’t manifest itself in the yoyo actually vibrating, but it can if the tracks are loose/damaged enough. Normally this is a vibe that you “feel” more than one you can actually see.
4) Bent axle
Again this one is pretty rare, despite many claims of it. It takes quite a lot of force to bend a steel axle, especially with the yo-yo fully assembled. The halves of a yo-yo are supported not only by the axle, but by the bearing. To bend a fully assembled yo-yo, you must also damage/crush the bearing itself. It happens, but not as often as a lot of people claim. If replacing the axle solved the problem, it could be one of the issues mentioned above… it doesn’t mean the axle was actually bent.
Thats’s about it as far as things that wouldn’t have been a problem to start with…