As mentioned there is a dialer included in VL which is less problematic than x-isp. if the modem was detected, log in as root and set up an account in chectnut..if it works then set access for userhttp://forum.vectorlinux.com/index.php?topic=9008.0
If you managed to get x-isp installed correctly then try typeing the name of the executable in a terminal or look for it in /usr/bin or /usr/local/bin..perhaps
I have not used x-isp. I use the included dialer . Any reason you need x-isp?
Have you seen this http://homepages.e3.net.nz/~keithmg/xisp.html
"Log into your user desktop and fire up Xisp from a xterm window so you can see error output if it occurs.
$ xisp &"