what's the difference between (vacant) and wildlands?
Vacant estates are available for any random noble wandering by to claim as their personal estate.
Wild lands are lands that are not assigned to any estate, and thus are not available for nobles to claim.
So if a lord does not want additional knights, they can either assign all land to occupied estates, or leave all excess land as wild. Since estates can be arbitrarily created, destroyed, and re-sized at the lord's whim, this is easily done.
what is there to do in estates?
Nothing at this time. There are future plans to add things for knights to do.
do they still influence region stats?
No. Estates only affect the collection of gold.
why would a lord want an estate if he can give it all away and just tax knights by equivalent amount?
Perhaps he can't get enough knights to fill all the available land? Perhaps he just wants one? Also, there will be some additional incentives added that may make estates more desirable to have. These are not yet implemented, and may not be implemented for a while.