It is only slavery if you judge it by human standards. Phlox describes it far more accurately as a symbiotic relationship. If the human perception is already flawed the treatment, human ethics, is flawed as well. I prefer Phlox's distanced humility to Cutler's and Archer's (initial) anthropocentric arrogance.
When humans and Vulcans met things started to change on Earth. But the Vulcans never helped us. They did not give us medicine to deal with radiation poisoning, they did not ship us food to deal with hunger, they did not prevent any violent conflicts. Millions of lives could have been saved!
They let us deal with all this stuff on our own. And when we finally got our shit together they did not want to share their technology! We had to work for decades until we breached the Warp 2 barrier while they just sat their and watched. In short, we were pretty pissed off.
And they were right.
OK, I am exaggerating, in this very episode T'Pol explicitly states that" Vulcans stayed to help Earth ninety years ago". I still doubt that the big breakthroughs like the eradication of hunger and war were done via Vulcan help. Earth wasn't united yet when they made first contact and before it wasn't united their hands were tied. Their job is not to solve the problems of another species and thus patronize it.
So much about what the PD implies concerning how to deal with one post-warp species. In Dear Doctor it's two pre-warp species.
I understand that people have difficulties with all this as the notion that stopping hunger, war or diseases might not be a good thing is counterintuitive. But intuition and moralistic instincts alone don't suffice to create a decent form of interspecies ethics.