"not the least" is usually used to mean that the reason given is actually one of the most important. "He had many faults, not the least of which was finding fault in everybody else." = "The fact that he found fault in everybody else was one of his own biggest faults - perhaps the most important."

"But even then Victoria stood out, not least because of her unflappable energy." Here, "not least" can be interpreted literally: It suggests that there may be a few reasons why Victoria stood out and her "unflappable energy" was one of the more obvious reasons why she stood out. An alternate version of the sentence with the same meaning might be: ""But even then Victoria stood out, mainly because of her unflappable energy.".

It is a reason, but it is not the least important reason.

Of all the reasons that made Victoria stand out, her unflappable energy was not the least significant reason. "not least because" = not the least significant