"vulnerable research subjects" are people who researchers collect data from who are in much weaker positions to say no and protect their own interests. These are people who are the focus of a research project who often cannot give informed consent to participate in research. They are generally in weak positions because for example they are poor, ill, under-aged (kids), socially disadvantaged etc. Other common examples include prostitutes, drug users, children, mentally impaired people, severely injured people, victims of rape/abuse, prisoners etc. Hence, researchers must be ethical in questioning such "vulnerable research subjects" as well as obtain a valid and approved human research ethics protocol and a protocol number. This ethics approval can only be given by an expert ethics panel based in a university, research institute or government regulatory agency. I hope that helps.
From my institution, the document https://ethics.grad.ucl.ac.uk/forms/guidelines.pdf details vulnerable groups, providing the summary "children and young people or other vulnerable groups such as those with a learning disability or individuals in a dependent or unequal relationship" but also providing further details throughout the document.
Vulnerable subjects in research on humans in 2018 will be based on human induced/ Human-instigated disasters. Examples include air/water/noise pollution, industrial accidents, oil spills and nuclear explosions/radiation etc.