the problem here is that your hdtv is unable to pick up the 240i signal the snes gives off using composite. You don't need a vga converter, what you need is one of 3 options (from cheapest to most expensive:
1. An upscaler. This will turn the signal to at least a 480i or higher. some force widescreen though so be careful
2. Scart to component/vga. You will need to buy RGB style scart cable for the snes (you are looking for the japanese 21 pin variety especially) This will then go into a Scart splitter and then into a scart to whatever converter.
3. Modify the console to natively give off component (This can be the cheapest method of the 3 if you can solder and have he parts lying around or someone is willing to do it for a fair price)
the reason why 2 and 3 can work is because the snes can give off 480i if given the proper connections to output the signal. however RF - svideo tend to keep things at the weaker signal. If you are not a purist though Just get a softmodded wii and maybe one of these:
http://www.retrousb.com/product_info.php?cPath=22&products_id=31