"We've become more targeted in whom we mailed letters to and more prescriptive in our language," McNabb said.
For example, Vanguard sent out 500 letters in March 2015 to independent chairs and lead directors outlining six principles of corporate governance, McNabb said.
Six Words
At the heart of the ValueAct case are six words in a 40-year-old law that requires disclosure of certain investments to assist the Department of Justice and the Federal Trade Commission in antitrust review of mergers. The intent is to prevent investors from secretly buying up stakes to agitate for industry consolidation.
The Hart-Scott-Rodino Act requires all buyers of voting securities worth more than $76.3 million to notify the government, unless they were bought "solely for the purpose of investment."
The government alleges ValueAct failed to disclose a $2.5 billion position in two companies that planned to merge - oilfield services peers Halliburton Co and Baker Hughes Industries Inc.
ValueAct is fighting the case and has said that its outreach to the companies was standard shareholder input and not active investing. The firm declined to comment for this story.
Among the evidence asserted by the government is a December 2014 meeting with the Baker Hughes chief financial officer, where ValueAct's chief executive discussed gaps in the company's North American margins and other underperforming areas. The government also cited a ValueAct email sent to Halliburton's CEO in July 2015 to schedule a meeting about executive compensation.
Davis Polk points out in its client memo that it's common for investors with stakes deemed passive to discuss those topics with corporate management.
"It does seem to be ... a typical subject of discussion," wrote the firm, which represents Baker Hughes in the merger.