xfce-4.6.1 is in the testing repository. To install it you need to enable the testing repository first. You can do this either from Gslapt, or by editing the /etc/slapt-get/slapt-getrc file(just replace #DISABLED with SOURCE for http://vectorlinux.osuosl.org/veclinux-6.0/testing/
Thanks! - That worked. Actually - it was the editing of slapt-getrc that worked. I had previously added that testing directory to the "Sources" Tab in "Preferences" of Gslapt, but I got some error message when I updated. I replaced #DISABLED with SOURCE, and all is well.