Once the biting was described to feel “painful but nice” but to me that sounds like hitting your funny bone except your funny bone is your major arteries.
The veins in the fang probably release some sort of chemical to make victims chill tf out because nobody wants to eat a dinner that screams at you.
As for the reason why only “virgin blood” can make a fledging… I dunno, I’d have to give actual though to it which I don’t want to do seeing as I have three brain cells.
Physically speaking, it’s really only a social construct. But vampires are social creatures so I guess it’s maybe a psychological thing. Thats really one of the only parts (along with Vicar’s plan) of Hellsing that I disagree with.
In the fanon version that exists only in my head, the fledgling/ghoul factor is just the intentions of the vampire.