The Vulcan Expeditionary Group was the Vulcan service branch comparable to Starfleet. Members might be trained at the Vulcan Learning Center and the Vulcan Science Academy. Before 2249, no non-Vulcans had been accepted into the Group.
In 2249, Michael Burnham, having graduated at the top of her class at the Science Academy, applied to join the organization. However, the director of the group, disdainful of Burnham's Human origins, offered Sarek (who was supportive of Burnham's application) a choice – between Burnham and his biological son Spock; only one could be accepted into the Group. Sarek ultimately chose Spock, and told Burnham that her application had been rejected. However, Spock elected to join Starfleet instead, which rendered Sarek's decision futile and caused Sarek to feel a great deal of guilt at being forced to make the choice (and over having misled Burnham about her application). (DIS: "Lethe")
Spock's preference to join Starfleet instead of the Vulcan Expeditionary Group caused a rift between father and son, and the two would not speak again for many years. (DIS: "Lethe", TOS: "Journey to Babel")