You may be able to, if the boot manager is on the Vista partition. However,
if it's on the XP partition, and you format it, Vista will be unable to
boot.
You can find out where your boot manager is by starting a Command Prompt
with administrator privileges, and then running bcdedit from the command
line:
C:\>bcdedit
Windows Boot Manager
--------------------
identifier {bootmgr}
device partition=D:
description Windows Boot Manager
locale en-US
inherit {globalsettings}
default {current}
displayorder {ntldr}
{current}
toolsdisplayorder {memdiag}
timeout 30
....
<<
The above output from my machine shows that my boot manager is on the
partition labelled D: in Vista (which is C: under XP). This means that
formatting my XP partition would erase the Vista boot manager, and leave
Vista unable to boot.