Index Ventures is quietly encroaching into the top tier of American investors. Here are five questions with them. There is a common investor behind several of this year’s biggest money-making events in Silicon Valley — a venture capital firm that has not historically been regarded as a tier-one player in the United States. But rivals and fans alike have noticed the year that Index Ventures, an investing firm born in Europe that has gradually encroached on the prestige of its top American competitors, has had in 2018.