If there are missing DLL files and the DLL files are Windows DLL files, you need to reinstall Windows. If the DLL files came with an application, you need to reinstall that application.
Yes, it should work to replace XP with Vista. Vista will put its own DLL files on.
As far as the hal.dll error mentioned by plonkeroo is concerned, many times if the boot.ini file is not correct, you will get that error message.