After getting yet another kernel panic after upgrading my VB, I've decided to further dig into the problem. On my testing it seems the problem might be related to the VB Web GUI. If I install a fresh copy of VB 2.0 from around 1 month ago and perform the upgrade directly via the command line with a simple yum upgrade, the upgrade goes smoothly and the system works fine. If I upgrade via the Vortexbox Web GUI, then I get the POSTTRANS error and respective kernel panic. Now given the package generating the error is httpd, it could be that whilst on the webgui the httpd daemon is active and, as such, somehow hampers the upgrade of the package and corrupts the kernel installation.
I have no idea how this could be solved but right now I've found the only way to reliably upgrade is via the command line.
Worth taking a look as I've seen already people moving away from Vortexbox after their systems get bricked everytime they do an upgrade. I will also look further into this and check back if I find a solution.