Make sure the stick is formatted to fat32 before you plug it in, when you have done this press the blue button on your remote and select Vix. Press the down arrow and select mount manager, in here you should see your HDD & usb stick and what they are mounted as.... the mount for the usb stick should be media/usb.
If the usb stick is mounted correctly then you will need to add your picon folder to it by ftp, when you have done this restart the box. To change where your epg data is stored go to menu/setup/system/epg/settings you can change the first setting for epg location to media/usb.
The most important part here is to make sure your usb stick is mounted correctly, Vu can be quite fussy about the make etc. Vix image automounts the hdd & usb for you so you just need to double check that it has mounted it to the correct location.