Creating a volume group for each distro defeats the purpose of LVM. Create one volume group and use lvcreate to create partitions for your distros. To avoid confusion, use the OS name as a label for the logical volumes.
lvcreate -L 2G -n gentoo-root
lvcreate -L 8G -n gentoo-usr
lvcreate -L 2G -n debian-root
lvcreate -L 8G -n debian-usr
Use one "real" partition for /boot that is shared between all distros and handle it manually and one "real" partition for swap. Use one logical /tmp for all distros, or use ramfs with a size of about 200mb for that. /home should probably be shared by all distros as well. Apart from that, I think that /usr and /var could be a separate partition for each, but not /var/log. If you have some special-purpose file-structures with many small files, than creating separate partitions for those may be useful too. In most cases though, It's really not necessary to worry so much about this. Especially when all you want to do is try out some distros. It's often far more convenient to create one root partition for each distro and be done with it. It's really up to the user how much complexity they are willing to manage.
separate logical volumes per distro
/ ~ 2gb
/usr ~ 4-12gb
/var ~ 1-7gb
/tmp (optional, this can be part of /)
/usr/doc (save space)
/usr/man (may be a bad idea for dissimilar distros)
/var/log (use syslog-ng to create distro-folders)
special purpose (many small files)
/usr/portage (gentoo "packges")