This video on grout cleaning recommends that you use only vinegar and water to clean your grout. As a professional tile and grout cleaner in Tampa FL I would not recommend using vinegar to clean your grout lines. The main reason for not using vinegar is that it's not a very good cleaner. It does tackle some base odors and is a mild disinfectant, but it does not clean very well.
The low pH does help break down some oils and grease but it's no more effective then lemon juice, which shares the same pH as vinegar.
Vinegar has a low pH and falls into the very acidic range. The acidity of vinegar is what gives it deodorizing and disinfecting properties. In short it can kill some bacteria but not most of the bacteria that makes us ill. The common cold virus for example is not killed by vinegar. Your much better off using an EPA registered sanitizer or disinfectant that kills up to 99.99% of bacteria.
So what would I recommend for cleaning your grout lines? I would recommend a mild neutral pH tile cleaner for regular use and an oxygen bleach for major cleanings. Oxygen bleaches like OxyClean are amazing grout cleaners. Despite the name they are not technically bleaches. They use oxygen to do the scrubbing of the grout lines. They're extremely effective and rinse away cleanly.