Are the Unversed Heartless?

Nope, not precursors. Heartless actually already exist at the time that Unversed first appear. Unversed are products of negative emotions/sentiment, specifically those of Vanitas'. They're like his minions, in a manner of speaking.

Is Vanitas a Heartless?

At first, Vanitas was a faceless being much like a Heartless but with red eyes, that felt sad for his separation from Ventus. ... When a newborn's heart came in contact with Ventus' fractured heart, he also directly contacted Vanitas' heart.

Whats the difference between unversed and Heartless?

The biggest difference between Heartless and Unversed in-universe is that Heartless are a manifestation of all aspects of the Darkness of a heart while Unversed are "only" negative emotions. Darkness as a whole is more than just negative emotions alone.