[villagetinker]

Try this, take some water and just place a few drops on the grout, if it beads it was sealed or has sealer built in, if it is absorbed, then NO sealer was applied or added to the grout. I have never heard of using vinegar for cleaning grout, we use ZEP PH neutral cleaner, works very well.
I am GUESSING the vinegar is slowly dissolving the grout.
I think you have 2 choices, first have the company remove all of the grout and correctly reapply new grout. This will be very messy. The second choice would be a color matched (to your grout) sealer. This appears to be a product similar to paint that is applied to the existing grout to seal the grout and maintain the color. I have personally used Grout Shield with excellent results. I did the whole house 2 - 3 hours at a time, moving furniture as necessary, took a couple of weeks. There is NO mess with this approach. Drying time is about 1 to 2 hours.
Hope this helps, good luck for your project.