Vector Linux 5.8 (Standard and SOHO) is based on Slackware 11. Anything built for Slackware 11 should (at least in theory) also work on VL 5.8, as long as all dependencies are met and it also works on Slackware 11. I've personally never encountered a package from a reputable source built for Slackware 11 that didn't work on VL 5.8. You should be able to enable Slackware 11 and linuxpackages.net sources for slapt-get or Gslapt. Another source which I've found helpful at times is www.slacky.eu
, an Italian Slackware site which has a lot of packages you might not find elsewhere).