What is the correct orientation of "ㄧ" in zhuyin fuhao?
Sorry for my misunderstanding of your question. Actually, because 注音符号 is abandoned in the mainland(it still remains in the dictionary), and is mainly popular in Taiwan, so I am not familiar with that system.

I searched 注音符号 on the Wikipedia, and I found the answer.


  依照中华民国教育部规定，注音符号的“ㄧ(yi)”在直写时要写成“—”、而横写时写成“丨”。台湾国语教育一般使用直写，故在台湾一般人并没学过“丨”，在横写时亦写成“—”。中国大陆由于废除中文直写，故字典等采用的注音符号字型，都固定造成“丨”的形状。下为瓶子两字的排列方法：
  


It says, the Ministry of Education of R.o.C specified that "|" was used when the characters were aligned horizontally, and "一" was used when the characters were aligned vertically. However, in the practice of education in Taiwan, usually only "一" is used and thus Taiwanese doesn't know the "|" form. In the mainland, only "|" is used.