I've found in place upgrades problematic with lots of distros. They work some of the time but often deliver a broken system. Slackware is hardly unique in this. I've had the same with Fedora and Ubuntu.
Vector Linux is NOT a clone of Slackware. Far from it. It is a derivative of Slackware. The majority of packages, including the kernel, are built to VL standards which are quiet different from Slack standards. Of course, VL also has a very different package list. If you look at what's happened with the 6.0 repo since the alpha releases started you'd see that most of the true Slackware packages have been replaced. The beauty of VL is that it builds on the strengths of Slackware (stabilty, reliability, speed) and then takes things to the next level. Anyway, that's my take on it. FWIW, I'd put some other distros (Zenwalk, Wolvix) in the same category as VL.