This looks like your first post in the VL forums. Welcome to Vector Linux.
The standard bootloader in VL is Lilo (Linux Loader) In my experience Lilo will automatically detect other Linux installations and add them to your list of choices at boot time.
I'm not familiar enough with Grub to say anything at all about it, though I do think it is available for VL 5.9.
So, during your installation of VL on sdc3, toward the end you will be given the options to install Lilo either on your master boot record (MBR), (which on your machine would be on sda), to a formatted floppy, or to not install Lilo at all. If you choose to install Lilo, it should automatically detect Mint, Klikit, and Xubuntu. Once Lilo does its autodetection, you should see 6 things in a list something like this: 1) Linux, 2) Linux TUI, 3) Linux GUI, 4) Linux-Mint, 5) Linux-Klikit, and 6) Linux-Xubuntu. The first three are for VL (TUI stands for Text User Interface).
Since Mint, Klikit, and Xubuntu are all members of the Ubuntu family, Lilo might identify them all as simply 'Ubuntu.' Don't worry about that for the moment (the Lilo configuration file can be modified after installation).
I hope this helps.