Easiest way to connect laptop with XP to laptop with Vista

It depends on what you are after. A network to share files, printers, etc. or you just want to transfer files. A USB memory stick is the easiest at first. But a network is the best long term solution.

You will need to connect each of your computers to an Ethernet Switch or via a Wireless Access Point or Router.

Then enable File and Printer Sharing and setup each of the computers on the same workgroup.