When I used Debian, I would sometimes port a package from unstable into stable via the source repository, which sometimes led to having to manually fetch dependencies. Or if I was installing a deb file that was not in the repositories, but was found on the web, via "dpkg -i", then I'd have to note any missing dependencies if it failed to install (I think they've made this easier now with the tool gdebi-gtk, which automatically gets dependencies for deb files that aren't installed directly from the debian repositories). However, here in Vector I find that even giving a readout of missing dependencies doesn't happen when I install a txz file. Instead, it simply installs the package, regardless of missing dependencies. So, I then have to run the program via the command line, and read the errors, to try to figure out what's missing. This includes both txz files created on my computer via vpackager and txz files from slackfind.net. vpackager and gslapt/slapt-get* are great tools, but they do sometimes seem to readily report packages as installed regardless of whether all the dependencies have been installed or not.
*generally gslapt does take care of all dependencies, but sometimes it will report a package as installed which still is missing a dependency or two.