Can I add XP after Vista (dual boot)?

There are several ways but they tend to be unreliable, and in some instances depend on your particular environment. Thie was one I experimented with, originally on rtm. There is no reason why it should not work with thretail version. I have Vista as my primary OS on C: and XP on D: (This is Microsoft's solution)

Note You can run the commands in the following procedure by using the command prompt. If you run these commands in Windows Vista, run them at a command prompt that has elevated user rights. To do this, click Start, click Accessories, right-click the command-prompt shortcut, and then click Run as Administrator.1. Use Bootsect.exe to restore the Windows Vista MBR and the boot code that transfers control to the Windows Boot Manager program. To do this, type the following command at a command prompt: Drive:\boot\ Bootsect.exe –NT60 All

In this command, Drive is the drive where the Windows Vista installation media is located.
2. Use Bcdedit.exe to manually create an entry in the BCD Boot.ini file for the earlier version of the Windows operating system. To do this, type the following commands at a command prompt.

Note In these commands, Drive is the drive where Windows Vista is installed. • Drive:\Windows\system32\Bcdedit –create {ntldr} –d “Description for earlier Windows version”

Note In this command, Description for earlier Windows version can be any text that you want. For example, Description for earlier Windows version can be "Windows XP” or “Windows Server 2003."
• Drive:\Windows\system32\Bcdedit –set {ntldr} device partition=x:

Note In this command, x: is the drive letter for the active partition.
• Drive:\Windows\system32\Bcdedit –set {ntldr} path \ntldr
• Drive:\Windows\system32\Bcdedit –displayorder {ntldr} –addlast

3. Restart the computer.