To my knowledge, the best Dreamcast VGA adapters are those from BeharBros. Their most basic model, Kuro, has an HD15 connector for video, RCA for audio, and lets you switch between 15kHz RGB (for 480i) or 31kHz VGA (for DTV 480p) depending on what the game you’re playing supports. The Hanzo model has the Kuro’s features, plus a scanline generator; and the Toro model has the Hanzo’s features, plus SCART output. They also have a newer model, Akura, that digitizes the RGB and VGA signals and outputs them over HDMI. (Personally, I’d go for the Toro, but you should get whichever one best fits your needs.)
As for Tomee, from what I’ve seen, you should steer clear; they don’t make quality cables.