Vulnerable vs Susceptible definitionThis is a key distinction in health science research, so here is the best definition I've found, and one I consistently return to.
From: Kottow, Michael H. "The vulnerable and the susceptible." Bioethics 17, no. 5‐6 (2003): 460-471. link
"The distinction between vulnerability and susceptibility also marks the difference between being intact but fragile – vulnerable – and being injured and predisposed to compound additional harm – susceptible. Awareness of this difference should give additional force to the rejection of double standards in research ethics."