The upper versions of Vista do offer a software raid so no prerequisite is needed to achieve raid. But and this is the crux of the matter, where are you going to find support for this? They don't supply a full version of Vista so your options are limited and expensive. That is, to get the software raid you'll be purchasing the full version of Vista that has that and then discover there are few people that will support it without asking for the credit card number.

If this must be done, I'd cancel the order and start over.