You can read it here.
Key points: The decision to pressure Zelenskyy to "investigate" Burisma and Biden predated the call, and involved Trump and his team. Vindman and others knew it was inappropriate, advised Trump of it at the time, and reported the action and its inappropriate nature to the lead counsel of the National Security Committee.
Vindman heard the call, and he and others who heard the call were concerned about the nature of the call at the time:
I did not think it was proper to demand that a foreign government investigate a U.S. citizen, and I was worried about the implications forthe U.S. government’s support of Ukraine. I realized that if Ukraine pursued an investigation into the Bidens and Burisma, it would likely be interpreted as a partisan play which would undoubtedly result in Ukraine losing the bipartisan support it has thus far maintained. This would all undermine U.S. national security.
They reported it at the time, again to the lead counsel of the NSC.
Trump's team is smearing Vindman on Fox News, from what I understand, and certainly on Twitter. Apparently they're very worried about what his testimony will do to them.