Are you still able to boot with the original kernel (2.6.11 I believe) for 5.1.x?
Did you go over all the various option in menuconfig (or gconfig or xconfig) to make sure that everything was set correctly? I know that is a pain to do but in your situation it obviously needs to be done.
When you ran
make menuconfig for 18.104.22.168 were there any errors or lines indicating non-existing options? There should've been a lot of them and sometimes you may find that very little is actually selected to be built. (If you just import a config file that is 11 subversions different and then build your kernel, especially a config from a different distro, then you will get a broken OS.)
Thus I would recommend that you restart the process for kernel 22.214.171.124 and after importing a config file that you know works with VL SOHO 5.1.x review the (hundreds) of options to make sure that all at least looks okay then rebuild your custom kernel. If you are not actually customizing anything and just want a 126.96.36.199 kernel then you could skip the config importing, open the working config file for VL 5.1 in an editor and make sure all the selected built-in and module options are selected. (This could be a bit confusing as there are many changes between 2.6.11 and 2.6.22. Also it may take a lot of time to do it that way.)
BTW what is the size of your customized kernels built with the slack 12 config and the VL 5.1 config?