The first leap by Vanguard into international small-cap waters through an index-based exchange-traded fund opened for trading on Monday. 

The Vanguard FTSE All-World ex-US Small-Cap ETF (NYSE Arca: VSS) tracks the FTSE Global Small Cap ex-U.S. Index. The ETF comes with an expense ratio of 0.38%, the cheapest in the field. It also holds some 2,100 different names, making it one of the broadest. 

Vanguard is also advertising it as the only broad-based ETF to cover both developed and emerging markets among international small-cap ETFs. 

The launch comes some two weeks after Vanguard opened the first share-class of the same fund. That was provided for mutual fund investors  (VFSVX) with a higher expense ration but the same index coverage.

Both VSS and VFSVX fill a big hole in the indexing pioneer's lineup. It's a gap that many asset-allocation-minded investors have been requesting be filled for years. (See related stories here and here.)