Does the pre-installed software on the purchased computer include Windows Vista, and is that something you actually need?

I apologize, but which half are you referring to? Everything has been working fine for a year and a half now.
But in general, yes, it is possible to do, the only downside is that the warranty is voided.

The warranty isn't always voided, and not with all manufacturers - that's the first thing.
Ideally, you can return the laptop without the hard drive.
Secondly, there's nothing stopping you from allocating a 10 GB partition for Vista and letting it sit there if that's what you want.

And thirdly, you were given misleading information. Salespeople like to do that - it's in their interest.