My understanding is that each distro and its affiliates share a package management system. tlz, deb, rpm, etc. the source code for an application is packaged to meet the requirements of the distro.
For example one distro may include application dependencies by default through the included apps and make available additional dependencies and include them, when required, by a downloaded application via the package manager. when adding applications outside this system, although the application may install correctly when compatable, it may require additional files which are not known until you run the application and errors occur.
although many distros use .deb,etc there systems vary and although compatable with each other, it is always best to look in the home repositories before looking outside for applications to avoid dependence issues. should something not be available Vector linux offers a place to request an application to help avoid such issues but there are times when you may not wish to wait and so should choose more closely compatable slackware packages or research requirements for building from source.
in addition to the default three repositories is a fourth called testing. applications built for vector go here first for testing and when felt worthy are transfered to the appropriate default repository. the default list can be updated to include any additions since you last used the pm or you can add the testing list and look for and test apps from there. it is wise to remove testing and update the package lists to avoid issues when updating the system. this may avoid installing or replacing librarys with untested ones by accident.
speed is often due to reduced complexity and may be found in any light linux. vector is fast yet includes all the required tools to build packages and includes all the generally used applications in addition to a lighter desktop while maintaining a slick appearance, so it is mid range. big distros include so much bulk that it slows them down and some of this is in the desktop environment like gnome and kde. additionally drivers,libraries, applications often not seen and generally unused add to the bulk aswell.
stable systems work hard to ensure that all the packages and dependencies and modules work together in all ways and any added application is compatable. this is always difficult and not completely possible with the constant changes in each aspect of the system. those that can achieve closely this state have a stable system. debian testing is an entire testing system that is old enough to be generally stable and is often used by other distros. debian stable may be stable (or close as it gets perhaps) but may not include all the more recent changes in an application or applications which may effect the stability which is more important.
Vector, although often overlooked, is very stable while offering many popular up to date applications. It is a surprise to many and if you like it feel free to tell others the good news
I am basically a new linux user who likes vector and this is just my thoughts on the questions. hope it helps