Particularly vulnerability to manipulation and/or psychometric tests to measure this construct? I'm looking for good places to start reading for an upcoming project. Thank you in advance.
Vulnerablity is the key concept for traditional trust research. Start to read the following articles and their subsequent works.
Mayer, R. C., Davis, J. H., & Schoorman, F. D. (1995). An integrative model of organizational trust. Academy of Management Review, 20(3), 709-734.
Schoorman, F. D., Mayer, R. C., & Davis, J. H. (2007). An integrative model of organizational trust: Past, present, and future. Academy of Management Review, 32(2), 344-354.
I understand your question to be related to suggestibility and not either positive or negative impression management.
In respect to suggestibility I have use the work of Gislih Gudjonsson and found his work to be helpful for court ordered evaluations. In addition to his research and textbook Gislih Gudjonsson had developed The Gudjonsson Scales published by Psychology Press of Taylor and Francis Group.
Infant vulnerability is secondary to illness early in life. When parents are told a child's life is in danger, even after recovery, they can perceive the infant as vulnerable and this can lead to later behavioral problems.