Vindman, who listened in on the conversation in his official capacity, would be the first whistleblower to have first-hand knowledge of the call.
Igor Fruman and Lev Parnas allegedly conspired to use foreign money to buy political influence.
The two married in Los Angeles last May after they had known each other less than a week.
Hunter Biden said he "did nothing wrong at all" in an interview with ABC News that aired on Tuesday, but conceded he may have provided an opening to attacks on his father.
"Hunter undertook these business activities independently," his attorney George Mesires said in a statement. "He did not believe it appropriate to discuss them with his father, nor did he."