Hello, I have a quick question – is 比甲 the same as, or similar to, 背心?
Yes and no. They both refer to sleeveless top (although depending on cutting it may seem to have a tiny sleeves when worn). They are interchangeable. The name 背心 was commonly used in Song Dynasty while 比甲 was used in Ming Dynasty.
In modern hanfu 背心 refer to 對襟通領無袖上衣（The first picture below)，while all the other types of sleeveless top are generally 比甲.