To explicitly designate a switch as the preferred VSF stack commander, first enable VSF on that switch (do not connect any VSF links yet!):
switch(config)# vsf enable domain 1337
This will reboot the switch. Once the switch has rebooted with VSF enabled, it should be member 1 as well as the de facto commander. Use the following command to set its priority to the maximum value:
switch(config)# vsf member 1 priority 255
This will ensure that this member will be selected as commander during any stack election (e.g., the stack reboots after a software upgrade or after a power outage). You can then add any additional stack members using the methods listed in the document linked by parnassus above.