If you already have a working copy of Grub, skip the Lilo installation in Vector. Then go to your (Mepis?) /boot/grub and edit menu.lst
I currently have two copies of Zenwalk plus one copy of Vector 5.9 on my laptop. The menu entry I use for booting Vector is as below:
title Vector Linux 5.9
kernel (hd0,11)/boot/vmlinuz-188.8.131.52 root=/dev/hda12 vga=791 splash=silent ro
There shouldn't really be any need to repeat the (hd0,11) section, but I've always found that a bit more reliable. Equally, the initrd is only to load the Vector splash screen on boot, so it can safely be left out. I use vga=791 on the laptop and vga=794 (1280x1024) on my desktop machines.
Its one of the nice things about Vector and some of the other Slackware-derived distros - they all allow you to skip installing the boot loader, unlike other "friendlier" distros....