If you install slackware on top of the ubuntu partition, you will lose ubuntu, but not windows vista. You may however lose the bootloader configuration that allows you to load vista. This is not a big problem though.
If you lose your windows boot option, edit the /boot/grub/grub.conf and add the following lines:
title Windows Vista
Note that the rootnoverify may not be necessary, but if you use it grub has a different way of determining device nodes. hda (or sda) == hd0, hdb == hd1, etc. So consider that when you update the bold section. The second number in the parenthesis indicates the partition, partition 1 == 0, partition 2 == 1, etc.