Hmmm interesting point Tinsel.
I don't think the Vampires needed to attack Buffys self-esteem, not really, she did that well enough on her own. At the end of season 2, after she had 'killed' Angel, she fell into a depression so deep that she just ran away from her friends and family; tried to pretend that she was someone else, just because she could not hack being the Slayer.
In season 5 after she returned from the dead herself her esteem was so low that she entered into a near violent sexual relationship with not only a man she hated, but a vampire (Spike). And these are just the two big ones.
Willow was afraid of Vampires to start with, but that diminished, probably from hanging out with Buffy and knowing that she was protected. In addition to that as her own mystical power grew, there was less and less for her to be scared of, as she became something to fear.
The later seasons were not as good as the first three, but that did not make them burdensome, there were classic episodes, good ideas and some great moments. They were much more character orientated which perhaps slowed them a little, but showed that the characters were maturing, moving on.
And the vampires, yeah well, they might have been a little more intelligent, but they were just monsters, human beings brought back from the dead without souls and a desire to drink blood. Most were just monsters for the hell of it, but we got to see more of a depth from Angel/Angelus, Spike, Drusilla, Darla and the Master.