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Actually, the title of your post is wrong: it is not about converting VGA to TV - what you want to do is to convert a composite video (or S-video) to VGA ..
I suggest you buy one!!! and the reason is just below (s-video-to-VGA circuit diagram) - you're not talking about a trivial item, you've got to not only decode the composite signal to produce RGB, you've also got to digitize the signals, store them in memory, change the number of frames per second and lines per frame, and play them back out.