Georgiana was in the vulnerable position of having no living parents, so her guardians (her older brother and her cousin Col. Fitzwilliam) weren't around all the time and had to rely on hired help (whether background-checked or not). There was definitely vulnerability there and it hardly matters whether it was recklessness or design - it was a near-ruin experience.

Lydia was in the vulnerable position of having no attentive parents, and her chaperones were friends who had no idea that Lydia would throw herself off to any artful man if not properly looked after.
A common denominator: a 15-year old girl - regardless of economic status - without being properly looked after by parents or relatives who really have their interest at heard can be potentially disasterous!