I was looking into this yesterday. The article doesn't state WHEN they first thought he was leaking classified info, just that "NSC officials attempted to remove Alex Vindman due to security concerns but were blocked by an Obama holdover, an official in the White House who still controls human resource management inside the NSC, an African-American woman closely tied to the Obama family."
https://www.thegatewaypundit.com/2019/11/breaking-nsc-officials-atttempted-to-remove-leaker-vindman-but-were-blocked-by-obama-deep-state-hr-head-in-white-house-letitia-lewis/