Holy smokes, this Alexander Vindman witness is very sketchy. Generally suspected of being “whistle-blower #2”, records show Vindman had numerous contacts with registered foreign agents, while a member of the National Security Council.  [FARA link – pg 4]
Additionally, it is highly likely Vindman leaked the content of presidential phone calls illegally while he was a member of the National Security Council; which explains why Adam Schiff would not permit Vindman to answer questions about who he talked to.

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The New York Times is reporting that sketchy Vindman attempted to manipulate the CIA transcripts of President Trump’s call with Ukraine President Zelenskyy to meet Vindman’s ideological interpretations. [Vindman had a hidden agenda “spying” while inside the NSC]
In an effort to bolster his very sketchy credibility; and likely in an effort to avoid the appearance of sedition; Schiff’s Lawfare staff recommended Vindman wear his military uniform to the hearing today, though Vindman never wore the uniform for his NSC job.

[Link to FARA document – Page 4]

(Open Secrets) – sA little known U.S.-based attorney quietly poured six figures into foreign influence operations for President of Ukraine Volodymyr Zelensky, hoping to be rewarded with a job in his administration, foreign agent records identified by OpenSecrets show.
The new Foreign Agent Registration Act records reveal previously unreported meetings with Trump administration officials and details of a six-figure lobbying campaign promoting Zelensky’s interests in the U.S. during the leadup to his election and now-infamous phone call with President Donald Trump.
[…]  Notable among the Trump officials contacted was Alexander Vindman, who oversees European affairs at the National Security Council. Vindman was one of five Trump administration officials chosen for a delegation to Zelensky’s inauguration featured in the whistleblower complaint alongside Kurt Volker, the U.S. special envoy to Ukraine who resigned after fallout from the whistleblower’s allegations. (read more)

President Volodymyr Zelensky and First Lady Olena Zelenska with U.S. Secretary of Energy Rick Perry and the U.S. delegation attending the inauguration, which included U.S. Special Representative for Ukraine Negotiations Kurt Volker, U.S. Ambassador to the European Union Gordon D. Sondland, the National Security Council’s Director for European Affairs Alexander Vindman and Deputy Chief of Mission for the U.S. Embassy in Ukraine Joseph Pennington.  (Source)