If Mepis uses GRUB as it's bootloader, like Ubuntu does (I could think so), you can copy Mepis' bootloader configuration before overwriting it, and compare that to the Ubuntu bootloader config after the setup if Windows' arent' added to the list automatically (they should be; every time I've done dual-boot -- never tried triple boot, but it shouldn't make any difference -- Ubuntu has correctly added Windows installs to the boot list). The file you need to copy (as root) is /boot/grub/menu.lst if you're using GRUB Legacy as you probably are, and after you've installed Ubuntu just compare that file with the Ubuntu's same file if needed. I think, though, that it's not necessary; I'm confident Ubuntu recognizes Vista (because the boot section for Vista is probably just the same as for XP and all the others, but just with a different partition of course). So in my opinion you can safely go on installing Ubuntu; if you like, take a copy of GRUB's config file. The same goes naturally for LILO if Mepis used that as it's bootloader, but the file would then be /etc/lilo.conf and after altering (if you alter) LILO config file you must re-run LILO for the changes to take effect.
If Mepis uses GRUB, copying and then comparing the config file is easier. Anyway it's not needed if I'm right.