EasyBCD isn't an
alternative to the Vista/7 boot manager, just an app for configuring it.
Adding a Vista/7 system to an existing Vista/7 system will dual-boot automatically, and therefore doesn't
need EasyBCD (though you can use it to custom-name your boot options etc if desired).
If you're going to add XP, that might cause you more problems, which you will be able to fix with EasyBCD.
Which of Vista and W7 is flagged "system" in Disk Management ?
If you're installing XP onto the
other HDD, then you'd do best to temporarily disconnect "system" while you install XP (after you've created the space for it).
That way the Vista/7 boot process will remain untouched by XP, and you can reconnect "system", put it at the top of the BIOS HDD boot sequence and boot.
You'll see your previous dual-boot and you can use EasyBCD 2.1 to add an automatic XP entry.
If you're putting XP on the "system" drive, it will regress the Vista/7 boot, and you'll need to
fix it with EasyBCD2 (after installing .NET 2.0 SP1 on XP), before adding all of the entries again.