No. VPC is not a boot manager. It is a virtual machine manager.
What prevents you from doing as you describe is that a virtual machine
running under VPC cannot go to the boot sector on a physical hard drive and
boot an OS directly from a partition on the host's hard drive. It cannot
even see any of the hardware on the host computer. The hard drive(s) it
sees are simply big data files stored on the host. VPC knows where these
files are but the vm does not.
VPC has its own set of emulated hardware, except for memory and the cpu.
The cpu is virtualized and the memory is allocated from the overall system
memory. The rest of the hardware a vm uses is emulated hardware. All of
this is managed by VPC such that each vm has no idea that it is not the only
OS running on the computer.
What you can do is create an image of the other system partition and use the
image to create a new virtual machine (just as you might with a real
computer). But in this scenario it is easier just to install the Linux
distro into a new virtual machine. The whole idea of VPC is to get rid of
dual booting in the first place. You use a vm instead of dual booting, not
to manage dual booting.
The cases where you would still want to dual boot instead of virtualizing
Linux would be where the work you are doing requires access to the host's
hardware (high end graphics or media content creation, for example). A vm
can only use the highly standardized emulated hardware provided by VPC, and
that emulated hardware is not very capable when it comes to specialized
audio or video work.
For a good read on virtualization and emulation see:
http://blogs.msdn.com/virtual_pc_guy/archive/2004/10/18/243821.aspx
There are important advantages to having standardized hardware for vm's
(such as portability). For some of the reasoning behind the choices of
emulated hardware see:
http://blogs.msdn.com/virtual_pc_guy/archive/2005/01/26/361361.aspx