I have a large jug of red wine vinegar that seems to have a good deal of sediment on the bottom of the container – can I still use it? It’s about a year old.
It doesn’t sound that bad. Vinegar itself consists of something that has already gone bad so there isn’t much room for advancement. It can get rank or skanky but you can just taste it to see if it is still suitable for what you wish to use it for. Vinegar can be kept for periods much longer than a year given some basic conditions.
heh…chemistry joke 
Sediment forms naturally in the less refined vinegars. It doesn’t affect usabilty, although you might want to filter when you get near the bottom of the bottle.
I’ve been meaning to ask. I’ve got some ancient rice vinegar in a sealed bottle that has turned brown (normally clear with a slight yellowish tinge). Has this gone bad? I assume that since vinegar is such an ancient condiment, chances are, even if it goes bad, it won’t kill me, right?
Right. Vinegar is normally too acidic for anything nasty to grow in. I supose you could hurt yourself if you kept your vinegar in a leaded glass decanter for months on end before pouring it over your salad, or left the top off the balsamic long enough for all the acetic acid to evaporate, leaving behind a neutral solution of sugars and proteins in which pathogens can grow. But either of those scenarios strain the bounds of the probable.
Maybe, with any luck, it’ll turn into wine.
I buy most of my vinegars by the gallon, so consequently, some of it is pretty old. Once in a while I’ll get a clump of “something” which may resemble a slimy sediment, except it sorta floats off the bottom. I’ve been told that it’s the “mother”.
Completely harmless. You can filter it out with a coffee filter and even use it to start new vinegar.
My salad thanks you!