I think it really depends. I think people view vulnerability as attractive for subconscious reasons. Through life we all experience things as existing a certain way so subconsciously in the present we make assumptions as to the nature of a person. That is why some people view vulnerability as being repulsive while others find it attractive. It is all dependent on a person's experience and what they want to believe that experience means. In the past I have gone back and forth. One time I would think vulnerability was attractive and the next I would think it was repulsive. One day I realized that a person's vulnerability or lack thereof does not truly depict what kind of person they are. That is most often the problem. People assume that vulnerability is accompanied by certain characteristics in a person. That is why people will list different reasons for being attracted to or repulsed by vulnerability in a person. Another issue is a person's values. If a person values vulnerability then they have subjective reasons for believing vulnerability is good. Or, in other words, the person thinks that there is something beneficial about the quality when in contrast to its opposite. This is all tied into what a person wants or likes. If a person likes taking advantage of vulnerable people then they will be attracted to vulnerable people. If a person feels empathy with a vulnerable person they will be attracted to a vulnerable person. If a person feels vulnerability is a weakness then they will be repulsed by vulnerable people. Some people feel the need to protect others because they empathize with them, some people feel the need to exploit others because they are thinking about themselves, some people feel that people should not be protected and should learn their own lessons so they can toughen up.
I do not believe that a man is attracted to a vulnerable woman because he feels the need to protect her. This does not occur anywhere in nature. This philosophy was probably written before studies were done on animal packs. Usually, male animals will be attracted to relatively vulnerable mates because they are easier targets. Vulnerability is relative. While one animal can be vulnerable to another in its own race it may be dominant to another within its own race. Animals do not usually find mates for the primary purpose of protecting them. Once they have mated and the female becomes pregnant then the male protects her because she is pregnant with its child. This has to do more with being territorial. It is more about self-interest.
Animals that exist in large packs usually protect the vulnerable either for the sake of peace or because they have some form of relationship with them. This does not mean they are "attracted" to the vulnerable though. It just means they feel the need to protect them for their own personal reasons. Usually, in nature, animals are not attracted to any kind of particular mate. They just look for mates that are in heat and are easy to dominate. This is for obvious reasons. In nature, if a female is stronger than the male who is trying to mate with her, she will most likely just beat the crap out of the male. Usually, in nature, a female is pretty resistant to a male trying to mate with her. The male animal, just like you and I, does not like to get the crap beaten out of it. It looks for easy opportunities. Animals also prey on vulnerable animals. It is all about ease of access.
Some females within species may seduce males during their mating period but then that is a matter of seduction rather than the male genuinely feeling a need to protect the female.
Last, animals in nature usually protect others because they are possessive (territorial). They are dominant over their territory and will fight off anything that tries to take what they have. When they protect things it is not usually out of a sense of empathy or compassion for what they are protecting.
According to nature the only thing I could see is that men are attracted to vulnerable women because they are easier. And once a man has dominated the woman he protects her because she is his possession. She is "his territory". Men usually stay away from women that will dominate them. That is not always true though obviously. Some men like to be dominated, even though that does not occur in the animal kingdom. You do not usually see male animals "wanting" to be dominated by anything.
Actually, some animals are not usually attracted to their partners because of their traits. They are different than humans in that aspect. Humans form decisions as to who their most beneficial mate will be and that defines who they will be attracted to. Animals do not usually care. They just look for any female that is in heat and that they can dominate. It is seen that some female animals actually select mates but I do not believe scientists really understand why they select the mate they do. It is commonly held these days that animals have a lot of sex with everything and sometimes babies occur because of it.