No one's really sure... I personally hope there will be a third Vegas, but most people will say that there will be NO more Vegas, since Ubisoft confirmed that they would not make another, although I found THIS link, which shows something hidden about a possible RSV 3 in the Ubidays 08 trailer which has been overlooked for a while.
"Something you might’ve missed on the Ubidays 2008 event back on May 28th was the below “10 Years of Tom Clancy Games” trailer. Not only does it briefly show past games, it also touts hints at unannounced upcoming Tom Clancy games with the line: “The Future Will Be Explosive.” The CGI footage shows Rainbow Six: Vegas 3, GRAW 3 (Ghost Recon Advanced Warfighter 3), Splinter Cell (Conviction?), and HAWX (High Altitude Warfare Experimental Squadron).
After 10 years of best-selling games, what’s next? Watch this Tom Clancy trailer to see for yourself.
As part of Ubisoft’s earnings report a month ago, the developer mentioned it had a pair of unannounced sequels in existing franchises set for release by the end of March 2009."