Because Vulcan was a hegemony, a sort of soft empire, at that time, with Earth under its heel. The launch of the NX-01 and its diplomatic missions established United Earth as a legitimate interstellar political actor, and undermined Vulcan's control over Earth's government. It represented one of Vulcan's client states essentially getting out of line, and Vulcan feared that other client states would do the same and that Vulcan would lose its position of dominance over smaller worlds.
In other words, Vulcan was upset at the launch of the NX-01 for the same reason the United States was upset with Salvador Allende being elected President of the Republic of Chile.