Well hello there. This experiment is right up my alley so to speak.
The yellow pack was probably Danstar champagne yeast, that's a pretty good/strong fermenter.
The red pack was probably Montrachet I think, not ideal for what you are doing, slower fermenter, less alcohol tolerance.
Vinegary taste may be acetic acid bacteria taking hold (acetobacter) which is generally not really good unless you're making vinegar.
At 9 days fermentation may have not been complete. These things are going to taste rough at best right out of primary fermentation, a minimum of 30 days aging is needed to make them barely palatable, and even then they'll be rough around the edges / fusel alcohols etc.
Splenda / artificial sweeteners I don't know, I don't like those they always seem to have a bit of bitterness to them so I never use them.