On the goal of the campaign reports only S1LSI. Other sources did not have enough political views and were not privy to the plans of the ruler.
The aim of the campaign after S1LSI was to return Tokhtamysh to the throne of the Golden Horde.
War erupted was an offensive on the part of Lithuania and defensive – on the part of the Horde.
Next to this the real (in my opinion) goal S1LSI proposed second goal: becoming the Khan Tokhtamysh should reward Vytautas authority over the entire Grand Duchy of Moscow and the whole Russian land.
I think this project is the result of Moscow’s political journalism of the early 15th century.
Full text of section – in the Ukrainian version.