Feature Name Install level
Feature_Core 1
Feature_Basic 1
Feature_AntiVirus 5
Feature_WebFilter 5
Feature_VPN 3
Feature_SSLVPN 1
Feature_WanAcceleration 5
Feature_EndPointNAC 3
Feature_Firewall 5
Feature_Vulnerability 5
Feature_SingleSignOnMobility 3

So long as the INSTALLLEVEL flag is equal to or less than the level of the feature. So if you wish to just install the SSL VPN you can use the following command:

msiexec /i <path to msi> /quiet INSTALLLEVEL=1