First thing is, see if you CAN purchase a new battery for your old PC. Laptops can change about considerably with model changes, and it may well be that the manufacturer does not stock the replacement you need. So you're stuck with hitting computer repair shops and EBay to see there's a machine similar to yours that you can strip a battery from ... This is not guaranteed to be a pleasant experience.
Other hand, if you can find a new replacement battery and the price is reasonable, why not? Vista's old but not embarrassingly old, so the machine will continue to be useful. And if you decide you want something newer, Windows 7 or a dozen versions of Linux ought to install quite easily.