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The Chinese wikipedia page for vertical bars only talks about:
In the book《成都方言》I'm seeing this type of xiehouyu with a vertical bar in the middle of the "注":
缸钵头的泥鳅儿——耍团转│滑不了
My best guess is that it meant either/or but then under the entry for 钵 it's written the same way:
钵 bo 钵钵，钵儿，陶盆：鸡吃一撮，狗吃一～。歇后语：“缸～头的泥鳅儿——耍团转│滑不了。”
No indications of different usage that I can see here.
What does the vertical bar do here? What does it mean? Is it either/or?
You are right, it means either or.
缸～头的泥鳅儿——耍团转
缸～头的泥鳅儿——滑不了
equal to
缸～头的泥鳅儿——耍团转│滑不了
There is no standard usage for vertical bar │ in mandarins. So, its usage is defined by the author of the text. You can usually find the usage key from the footnote written in the early pages of a book. For example, │is used in 现代汉语词典 to separate examples. Like this:
[蹉跎] ...... 岁月~ | 一再~
So, you can define your own usage based on your need and you might want to provide some usage note as well.
