I've created some books, and I want vendors to sell them instead of me handplacing them in God know how many cells. What is "VendorItem" in keywords? I've given an existing book a new ID, new content, so it carried with it that keyword.

According to Wiki it says VendorItems allows those items in your inventory to be sold to merchants, but does that also mean items you create and have the vendoritems keyword will allow merchants to stock it as well? Or will I have to place them in the vendor sell levelled lists so the player can buy them?

VendorItem usually pertains to items you can find inside a Vendor mainly, keywords are great for describing an object incase you want to use "HasKeyword" function etc, to see the classification is equipped or in your inventory