背心 is not a cloth like vest or t-shirt, etc here, but is middle of one's back. the position is usually called 后背心 or 后心

Wow,me too. As a Chinese,I can't under stand '背心没有肋骨保护',I guess those means are '背心没有保护'? Someone is so powerful,he can kill someone by fist.

"The wooden door struck the pirate right in the back. (Unlike the chest,) a person's back has no protection by the ribs, and the strength of this single punch was unfathomably powerful. Right as the punch lands on its target, it inconceivably blasted straight through him." But in this context, it sounds like the paragraph is describing someone punching a pirate on the back. So I'm inferring that 木门 is some person's name? Or maybe someone was punching through some pirate leaning against a wooden door.

The wooden door(I'm not sure due to poor context, it can also be a name) hit the pirate at the back, where there is no protection from the ribs. The strength of this bunch is so strong that is out of people's imagination, it even hits him right through.