I may have been responsible for one of those remarks, this Vista stuff is
all new to me too, so can't tell to what extent the Vista installer will
react or even care. But in more general terms, when installing, the
installer is well equiped to handle whatever - as long as you don't throw a
Linux installation at it, but then the Linux installers would be sure to
handle it. The installer will not care what created the partition, it will
know if it is bootable and if it is empty.
I was surprised to see the Vista Partitioning routine did not tag a name or
any designation to the partitions - only the type and the size and the
partition number, so you have to be able to recognize your partitions by
these designators in order to know what to install on and what to leave
behind.
As long as you can point the installer to the partition, it doesn't matter
if you delete it first or if it was PM that created it. If you go ahead and
point the installer to the PM partition however, I cannot be certain at the
moment that it would set up a boot menu for you, so you might loose the
option to boot into the first system.
If I have Andre correct, your best option now would be to use the Vista
installer and delete the PM partition there, then create an extended
partition, and create a logical partition inside that (to give you a drive
letter) inside the extended partition you can create more logical
partitions - if you divide up the space between them, but any space you
leave behind will not be visible to the OS, and you will have no way of
getting at it from inside the OS, so however way you want to put this space
to use, is determined at this point. But using the logical partition, the
system should set up a boot menu for you and give you an option to boot into
whatever.