You can change the vSphere 5.1 Web Client Frontend port post-install by editing the following config file on the Web Client server:
C:\Program Files\VMware\Infrastructure\vSphereWebClient\server\config\tomcat-server.xml
Find the entry below and edit the port accordingly:
<Connector port="9443" protocol="HTTP/1.1" SSLEnabled="true" maxThreads="500" scheme="https" [...]
Restart the Web Client service afterwards for the change to take effect.