No, vendors don't carry lightstones as part of their permanent inventory. They often have some that players have sold them. A Vendor charges about 11pp for a Greater Lightstone.
Regular Lightstone: When you turn them in to the gypsy vendor, she'll give you a research book (not the concordence). The books are grey and called "Runes and Research, Volume I and Volume II." They're just instructions on how to make practice runes for spell research skill ups.
Unless you're a spell researcher, you're better off selling your lightstone to a vendor for the 8-9 gp.
Greater Lightstone: Turn it in to the appropriate gypsy in exchange for a "Concordence of Research". This is a container so make sure you have an empty inventory slot! Sell the concordence back to her for about 9pp. This is only profitable if you're obtaining your GLSs for less than 9pp, obviously.
Is that what you're trying to figure out?
By the way, the name of the gypsy in Oasis who takes the Greater Lightstones is Synthan. I don't know the names of the gypsies in the other zones.