This was discussed in some detail in the initial VYB white paper.
The major difference is that VYB rewards are not affected by downvotes.
Also, I like to think of VYB as a 'companion' token to POB, but with an explicitly long-term focus (i.e. as a retirement / rainy-day token).
Hmm, sounds interesting. So some flaws were observed in POB then VYB was created to correct that? That's how I kind of see it.
I would not call them 'flaws'. POB was launched as a bold experiment and has been very successful. VYB is a new kind of experiment, that has the benefit of some of the lessons learned via the POB experiment. The two are complementary, like yin and yang.