Here's my general understanding: GameCube was not designed to support non-interlaced VGA output. VGA adapters exist as an attempt to let you play GCN games on a pc monitor. One of the main goals of VGA conversion is to allow you to see your p.scan games in p.scan w/out having an "expensive" TV. But you have to keep in mind a certain condition: pc monitors ONLY display in p.scan (which is the same thing as being "non-interlaced"). So, a given VGA adapter will be one of 2 types: 1) It will "pretend" to be a GCN Component Video Cable and let you see p.scan games generally fine, w/ or w/out a few minor issues that you can find mentioned in some reviews. The main downside is, non-p.scan games will show as out-of-sequence scanlines, as opposed to clean frames, which are only meant for TV display in the first place. OR, those non-p.scan games won't show up AT ALL or be an even bigger worthless mess of video.
2) The VGA adapter has deinterlacing functions in an attempt to re-organize the scanlines, and convert those now-progressive frames to VGA format. It may or may not matter if the GameCube is allowed to turn-on p.scan at start-up, which would occur in option #1. Now, if the GameCube's p.scan function is not compatible with this VGA adapter, then all the GameCube's video output will be interlaced/non-p.scan. In that case, 30fps games (ex: Resident Evil, Zelda:WW, Mario Sunshine) will probably survive the deinterlacing/conversion, and look decent on the monitor. But for 60fps games (ex: Metroid Prime, Mario Kart), the adapter will either force it to run at HALF the framerate (30fps) and half the detail (like it went thru a cheap Photoshop image resize), OR the game will still run at full 60fps but still at half the detail (cheap image resize effect again). Depending on the quality of the deinterlacing function, scanlines might still show up.
VGA conversion is not a simple task, and is "never" totally successful; there's definite sacrifices in quality somewhere. This is my opinion.