The Ichiba spacers are usually hubcentric. The little "lip" around the center of the spacer is the hubcentric ring; it's not a separate piece like Project Kics spacers. The hubcentric ring is there to center the wheel on the hub; when you don't use hubcentric rings, the lugs are used to center the wheel which can cause vibrations at speed.
Check to make sure that the hubcentric ring is still intact and also that it fits your wheel's center bore properly. If there's any kind of play, this is probably where the vibration is coming from.
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