... RCVs can also apparently choose who they want to feed upon until they die, and those they want to half-turn. Bianca's secretary was not a vampire, nor was she half-turned, and apparently Bianca did sip her blood from time to time. The secretary apparently went along with it because whatever it is in RCV venom, she felt narcotic pleasure and so did Bianca ...
Yes, Harry himself has experienced the narcotic pleasure of Rampire-venom (several times), and attests to the intense and immediate euphoria & loss of judgement.
Rampires -- at least the higher levels of them -- often had "harems" or "herds" from whom they eat lightly but widely, keeping them all addicted to the pleasure but seldom taking any of them to the half-Ramp stage, nor killing them.
... when things went to hell many were killed outright and others like Susan were half-turned. Then it seems to come down to will, some join the resistance and others go on to kill and become full vampires.
I think feeding-until-dead can be deliberate, or from loss-of-control. It's less clear what causes the "half-turned" phenomenon... does it have to be intentional? Does it happen automatically, unless suppressed? Etc.
What a newly-turned half-Ramp does with their status is highly-dependent on a bunch of different things.
People like Martin were longtime (human) servants of the Rampires, and were deliberately half-turned, then coached / supported / controlled through their initial bloodlust (eating lightly but not killing) until they could control it themselves. I think the Fellowship of St. Giles does something similar, for newly-turned who come under their influence.
Harry managed to coach Susan enough that she could hold it off, but she wasn't able to consume any blood to sate the hunger, so the urges were getting worse and worse. We don't know how Susan managed to get her hunger under control after the climactic battle in
Grave Peril, before she left Chicago. It seems very likely to me that the Fellowship contacted her there in Chicago, in the aftermath of the Velvet Room burning down (they likely had Bianca's under surveillance, because of all the Red Court activity surrounding her elevation to the nobility; they might have picked up Susan's trail there). This is yet another open question in the series.
It's also very possible that -- in all the carnage after Bianca's battle with Harry -- Susan was able to sate the bloodlust from the freshly-dead, not have to actually perform a killing. That might have given her the time she needed.
... I don't think that every RCV started out as human and was half turned, is there a word of Jim on this? ...
That's just how it works:
A human has to get infected/implanted with a Ramp-spirit before they become a full Rampire. At that point, they're half-turned.
They may stay in that state indefinitely, or they can turn full-Ramp very soon; but AFAIK they always have that state of being half-turned: there's no way for a human to go from "just human" to "full Rampire" without -- at some point, however briefly -- being a half-turned who performs a killing.
I don't think we have WoJ on how short that half-Ramp stage can be, but I think a single day or so is likely (it might be something specific like "until the next nightfall" or the like, meaning that someone turned just before the deadline may be able to go full-Ramp in just minutes, while someone turned just after the deadline needs to wait almost 24 hours... but this is entirely speculative, I know of no canonical or WoJ info on the point).
... Nor do I remember exactly where is says that her promotion gave Bianca "permission" to turn and half-turn humans... Or did her promotion merely give her authority to choose who she wanted to recruit and who she wanted merely as food? ...
I think we infer quite a bit about Rampire political and power structures from bits & pieces here and there. I don't recall any specific canon/WoJ statements as to what Bianca's new powers would be, and thus don't know whether they were mostly "legalistic" permissions, or were extra Ramp-mojo magic/psychic stuff, or both.
...Another interesting point if I remember correctly from the party, one of the things that Harry and Michael discovered was that the wine was either drugged or poisoned, so many of the young recruits weren't even considered suitable as food.
It was a warning from Thomas: there was Rampire-venom in the drink.
Michael hadn't been drinking, I think; and Harry forced himself to throw it up.
I think the human servants & "party snacks" circulating at the party had been being recruited for some weeks -- maybe even months -- by Bianca, so she'd have a lavish offering at her ball. Plus of course her longtime servants, who I think were largely tasked with managing these new recruits. I think there was a mix of down-and-out homeless off the streets (meant largely as "just food" for indiscriminate eaters like Blampires) with "rebellious (but attractive) youth" from local colleges &c (for aesthetes like Whampires).
