This is not what we want to hear.
If it tastes of vinegar it sounds very much like you have got an infection in the wine. The normal cause is at the end of fermentation something might have got into the brew (like a fly) which will have caused this. This will happen when the wine is hanging around waiting to be degassed, fined, and bottled. If we think about it we have a container that is too big for the amount of wine we are making (we need this at the start due to extra space for the fermentation).
Whilst the wine is fermenting it’s giving off CO2 so no danger of getting an infection at this stage. But once the fermentation has finished any headspace on the container is a potential trap for flies and the like that can cause infections.
We recommend that if you are making a wine with a slower fermentation (normally a more expensive type) that we start in a bucket but when the fermentation has slowed down (typically the gravity is down to 1010) we transfer to a 23 litre carboy (this has very little headspace and therefore little chance of picking up an infection).
We can degas and fine in this but if we leave it for storing the wine, we should always top up with clean water to about 50ml (2”) under the bung. This way we are avoiding any headspace for the wine to get infected. If we are doing a quick wine (less than 10days processing) then this is not so important as the wine won’t be hanging around (unless you don’t do the very processes when you need to).
The yeast is also important as we find that it can take up to 3 days before it starts. We need to make sure the temperature of the liquid when this is added is correct and in line with the wine kit instructions (or making a fruit wine in line with the instructions on the packet of yeast). If the temperature is too hot we can kill the yeast either the whole sachet or just parts (so when we need it to grow during fermentation there will only be a % of the packet that is live and will do this on a limited basis). If it is too cold, then it won’t start to work for quite a time.
The viability of the yeast is important as if it's old then the chances are its not going to work whatever the temperature. It's also very important to make sure we have the right nutrition (to feed the yeast with the right minerals and vitamins). Most quick wine kits will have this mixed with the sachet of yeast.
Cleanliness also has a big part to play. Sterilisation at the start and when we come to do any processes on the wine is paramount.
Is the wine in bottles? If so, is it every bottle? Do they all have the same taste as its quite common some bottles (especially if they are used) don’t get cleaned as well as others (or have a hard crust on the bottom making it difficult to clean) so you might well find it’s not the whole batch.
So hopefully there is a pointer that might have caused your problem. In all my years I have to say it is a very rare occurrence that it’s the kit that is faulty nearly always a yeast problem or air infection.