I am dual booting XP/Vista and Windows 7 beta and it is fine. Just create a new partition by following this here click here.
ALWAYS back up before doing this.
Once you have created your new partition (give yourself about 20 to 30gb) you can then install the beta on the blank partition.
If you are unsure about this then research it before carrying it out because you wouldn't want to lose any data or mess up your Vista install.
I am sure you will get any help you need on here if needed.