What is the tosfos yeshanim? It is in brackets and small letters in the vilna shas. How do they differ from the regular tosfos?
There were many versions of tosafos on most tractates. Starting about the time of Rabeenu Tam and his nephew R"I Hazaken -R Yitzchak of Dampierre, many of the senior talmidim of the great tosafists in france compiled their masters "tosafos" on various tractates, often adding their own remarks as well. These were variously referred to as tosafos of the master or the pupil. Some of the later Tosafists (such as Mahr"M RoSh and R eliezer mtuch) compiled versions of Tosafos themselves, with the purpose of giving over the teachings of the R"I and his great talmidim, their own comments only added "אגב".
When tosfos was added to the page of early editions of the talmud, the particualr tosfos printed depended mostly on what was available to the publisher. Later Achronim often had other versions of Tosfos in manuscript. These were sometimes referred as "Tosfos Yeshonim" as in old manuscript of tosfos. There is no connection between tosfos yeshanim for one tractate and tosfos yeshanim on another. Authorship must be determined for each one individually(often there isn't an easy way for it to be determined definitively). Some of the tosfos yeshanim manuscripts were eventually printed on the margins of later editions of the talmud. While the tosfos yeshanim were originally a full version of tosafos on that tractate, the printers often only included small glosses which they consided to be substantially different than the tosfos already printed on the page, which is what you are seeing on the margins in the Vilna Shas.
Hope this was helpful.