Is there any way to know what avatar ID they are using from their VRChat user ID?
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Yes and no. If I understand correctly, if you use VRCX, which you can get from here if you are in a lobby with someone and their avatar shown, you can sometimes see the avatar ID of the avatar they're wearing, and if they have vrc+, then you won't be able to see there avatar name or ID. hopefully that answers your question
