I'm not one of the developers or packagers, but ...
1) Have you looked at the development thread for VL Standard 64-bit: http://www.vectorlinux.com/forum2/index.php?topic=3454.0
? The thread gives a pretty good idea of how 64-bit is going for VL. Whether or not there will even be a 64-bit VL SOHO might depend on how well the Standard version goes, and whether or not users want a SOHO version.
2) In the mean time, have you tried VL SOHO 5.8, or VL Standard 5.9 for 32-bit (you can install KDE on it)? I also have a 64-bit architecture, and VL 5.9 (32-bit) is very fast on my machine. With your specs I think you'll probably find it much faster than Fedora Core 8 64-bit.
3) In my experience, VL is the fastest Linux distro there is that isn't stripped down to the bare bones (a la Damn Small Linux or something like that).
4) I don't know what your needs are, so I don't know if VL in any form is right for you. Probably the best way to find out would be to free up a partition and give it a whirl.
All the best, regardless of what you end up using,