Yes, you can "completely replace" any operating system with any other one. So go ahead and install a Linux now, and later when you get Vista you can install it the same way as if you never even thought about Linux. Or maybe you will want to keep it in addition.
You can mess up partitioning and you can mess up a second installation (losing the ability to boot the first OS until you repair the boot manager), but those errors can be fixed. So, the risk is in spending more time and effort than you had planned and hoped.
There is plenty of information about dual booting and installations. Here is a recent thread on
triple booting with links to a lot more info.