Vladimir-Suzdal (its capital quite close to OTL Moscow) was one of the strongest states in Russia, dominating the north-east. In TTL, the horrible attacks of the Mongols 1237-40 never happened of course.
In 1238, the Russian prince Aleksandr Yaroslavich (OTL Nevsky) who was the fourth son of his father and had no chance of ever becoming a ruler went to Vladimir-Suzdal, who were often busy fighting the Volga Bulgarians at this time. After being defeated by Aleksandr 1253, the Volga Bulgarians had to allow the Russians of Vladimir-Suzdal to go with their ships on the Volga without harassing them. This helped Vladimir's trade down to the Caspian Sea, with Choresm. After the death of Aleksandr, the Russians founded 1295 the city of Aleksandrskoye at the Volga, at the site of OTL Kazan, also to control the Volga trade better.
During the 2nd half of the 13th century, Vladimir-Suzdal went on to control the thrones of the East. In 1283, they finally overwhelmed Ryazan, its old challenger.
In 1334, Vladimir-Suzdal declared that the metropolitan of Vladimir was the highest authority for Orthodox Christianity. Not everyone agreed with them, though.
The Teutonic Order conquered the Russian princedom of Turov-Pinsk in 1348. Part of the people fled and ended up in Vladimir-Suzdal's sphere of influence, settled in the areas formerly inhabited by Volga Bulgarians and Volga Hungarians.
Since ~1350 Vladimir-Suzdal had started to expand into the woodlands between Volga and Ural. The Muslim and pagan people living there were converted by force and assimilated, or killed and displaced.
Things were shook up again in the 1380s when the steppe people leader Arik-Buqa drove the Kara-Kitai and other Siberian people west, so they harassed the Volga area. The eastern Russian princes made Vsevolod V Yuryevich of Vladimir-Suzdal their leader, to fight them. But around this time, in 1383 the patriarch of Kiev crowned Vladimir V Czar of all Rus. The difference between the South / West and Vladimir-Suzdal that already was apparent became even more outspoken.
As a consequence, in 1388 the Russian Schism began, when the metropolitan of Vladimir and the patriarch of Kiev excommunicated each other, driven by the Czar / Grand Prince of Vladimir respectively.
1399-1402, the Black Death reached Vladimir-Suzdal and the other eastern Russian states.