My experience with those cables has been lousy. The Wii does not have VGA natively, and this cable uses a chip to convert component to it. From what Ive heard, the chip does not have the supplementary components it needs to work properly. The cable has issues. It will flicker black sometimes, wash out colors, and have trouble with interlaced video signals. 4/10, would not recommend over composite. There really isn't a good way to convert Component to VGA in a portable. Which really, really sucks because most small screens use VGA.
If your Wii portable is going to use a 7 inch screen, this is a great screen that accepts component video. We have yet to find anything (reasonably priced) that is smaller than this screen and accepts component video. If you want something smaller, your choices are Composite, S-video, or GC video.
GC Video is your best bet for VGA on a wii. It is wired straight to the GPU data lines, and grabs the video before the wii's video encoder. Its a bit more wiring, but you will get unrivaled picture quality and VGA for all your small screens. Problem is its still kind of in the "experimental" stages. SS and Aurelio got it working, but have yet to finalize and release their code. SS is the only one selling GC Video boards programmed for wii right now, so if you want one, you should contact him.