If your rapidinstall packages are written to explicitly check for Windows XP as the operating system, then you will not be able to run these packages on Vista without resetting the system requirements on each package.
There is ONE possibility of working around this, which I have not tested. However, vista allows you to set the properties of a shortcut to an executable, so that the exe perceives an older operating system when it starts up. Whether you can set up a similar "emulation" for a rapidinstall package by setting up a shortcut to it, with XP as the "faked" operating system version, is something you will have to try for yourself.
However, this approach may be OK for a single installation, but would not be suitable for a corporate environment.