How do I modify a virtual server's max clients (max concurrent user connections)?

Login to the ServerQuery from within your client (Tools => ServerQuery) and use the "serveredit" command to modify the properties of a virtual server.

TS3
login serveradmin password
error id=0 msg=ok
use 1
error id=0 msg=ok
serveredit virtualserver_maxclients=64
error id=0 msg=ok

Please check the ServerQuery documentation (ts3_serverquery_manual.pdf) for additional information on how to use these sorts of commands.