Vinegar rinses are supposed to lower the pH of your hair and scalp. They're supposed to be slightly acidic, and dilute vinegar supports this. It closes the cuticle of your hair to make it smoother and shinier, and it's a virtual panacea for tons of scalp problems.
It won't dissolve your hair, but too strong isn't going to do you any favors. Some people do complain that it seems to make their hair seem greasy or lank -- if that happens to you, try diluting it a bit more. Taste-test your mix. It should be mildly sour, not gaggingly so. Some people use as little as a 1/100th dilution, while others go as high as making their mix 1/5th vinegar. I personally suspect that this has something to do with the local water -- mine has high alkalinity (buffering capacity), so I need quite a bit of vinegar to get the mix to taste at all sour.
Rooibos tea? You can do anything you want -- it's your hair! Mine's not fond of tea, especially if it's not rinsed out afterward, but I know lots of people love it.