VNET [21Vianet] SC 13G/A: SCHEDULE 13G (Rule 13d-102) (Amendment No. 1) 21Vianet
[SCHEDULE 13G (Rule 13d-102) (Amendment No. 1) 21Vianet Group, Inc. (Name of Issuer) Class A Ordinary Shares, par value US$0.00001 per share 90138A103 (CUSIP Number) December 31, 2013 (Date of Event Which Requires Filing of this Statement) ¨ Rule 13d-1(b) ¨ Rule 13d-1(c) ý Rule 13d-1(d) Page 2 of 7 Pages 13G CUSIP No. 90138A103 1) NAME OF REPORTING]