If Vector runs on the PPC architecture, I would second that motion. I run Vector on an older PC of mine and it works very well.
I would stay away from YellowDog Linux, which is specifically a PPC release. Very, very poor support - I couldn't even get registered on their forum for pete's sake. I just gave up.
Ubuntu is overrated, but that doesn't mean it is bad, just that it is getting a lot of hype. You will likely find that it is the most CURRENT of the available PPC distros and hence the smoothest install. Vector is good, but be ready to tinker with it a bit. Also, with Vector, you get XFCE or IceWM as your defaults. These are lightweight and fast, but they may feel a little sparse to you. Ubuntu comes with Gnome, which is a more featureful interface. There is also Kubuntu, which is the KDE version of Ubuntu. I am not sure if it is PPC though...