Well you obviously need at least 1 partition for each different distribution. For reliability's sake you should ideally break up a Linux system into multiple partitions, but if you are just testing distros out and aren't trying to setup a permanent desktop you can get away with one partition each.
Additionally, you should create one swap partition that both distributions can share. This would leave you with 4 partitions total, which will work out well since you don't need to use any extended partitions. You will have one for Vista, two for different distributions, and the fourth for swap (this only needs to be 256 MB or so).