Question : How can i search for Vanguard ETF's (Exchange Traded Funds)  on SEC (Securities and Exchange Commission) Search tool EDGAR?

Answer : ETF's can be searched under the Mutual Funds search  of EDGAR.  EDGAR documentation at https://www.sec.gov/oiea/Article/edgarguide.html quotes : "You can use the Mutual Funds tool to also search for ETFs. You can search for other types of funds as you would search for public companies."

Searching for Vanguard ETF's in Edgar. There is a trick to it. Vanguard ETF's can be found as 'Series' under a master Vanguard CIK. 

As an example, searching for the famous  Vanguard Information Technology Index Fund  with it's symbol 'VGT' does not bring back any results. This is because the ETF is registered as a Series under CIK #0000052848 for 'VANGUARD WORLD FUND'. Hence, Search for this ETF with it's name rather than it's Stock symbol .