There is no fix date when the Goth split into Ostrogothi and Visigothi. The terms itself are from different times or at least reported in different times. The first time the term Tervingi appeared in the sources was 291, those of the Vesi in 391/ for the years 388/91. The greutungi name appeared 369 and those of the ostrogothi in 399.
The reasons are more geographically. While the tervingi lived in more wooden regions, the Greutungi populated more the Steppe zones.