Assuming that you cleaned out the fermenters properly and keep the airlock filled (i.e. keep everything sanitary and bug free), you shouldn't have a problem with the wine going over to vinegar.

If this ferments out to completion, you could get specific gravities in the .994 to .998 range which would give you a very dry wine. In this range, your alcohol content will be in the 12% range - and this will be fine for keeping. If if stops around 1.010, most people would consider it very sweet but it would wtill be at 10.5% alcohol and some commericial white wines are bottled at this level (though they're usually consumed within a year).

Right now, I'd let the wine continue to ferment and see where it goes. Once it almost stops, I'd take another reading to see where you are in terms of both alcohol content and sweetness. I'd also give it a taste to see if it's still like grape juice - which yours won't be - or in the range of sweet or dry that you like. You can always add sugar to sweeten the wine ... just remember, there's a good chance that if you do the wine will keep fermenting until either all the sugar is gone or the yeast die from alcohol poisioning. Do it this way and you'll end up with one potent wine - to just raise the sweetness without the alcohol content, use glycerine.

Remember, the cloudier the wine is the more yeast you are going to taste so let it sit a week or so at least after it stops to get a really good taste of what it is. As you do more of these and move to varietal concentrates in cans, grape juice and maybe even fresh grapes you'll want to taste when the fermentation is done and at each racking (every few months) to see how the taste changes and when you feel it's best to bottle and serve.

All that being said, I don't believe you are going to experience any of that or get sick from this batch ... your fermentation has gone from pure must to around 8% alcohol in about a week which is going to give you a lot of protection against bacterial infection and it's still going so as long as you keep things reasonably clean, you're going to be fine.

Don't throw it out, even if it gets no better it's perfectly fine as a marinade or to use in other cooking. You now have options as to what to add and when:

1) You can add more sugar at this time and the fermentation should restart as it eats up the new food. How much is a bit tougher to tell since everyone's tastes vary. What I'd probably do is add about 1/2 lb to 1 lb of sugar to the batch, stir it to mix well, take a new specific gravity to see where you are and then let it go again. What should happen is the specific gravity will go down again as the fermentation continues and the alcohol will increase until the yeast reaches its alcohol tolerance and then for each time you add sugar, the gravity will only go up.

2) Add some more concentrate in. This will also add sugar and the alcohol level will increase but it'll raise the final specific gravity since, unlike sugar, a portion won't ferment and this will make the taste more pronounced.

3) Wait for the fermentation to stop completely, transfer it into another carboy and let it sit until pretty much all the yeast is out of it, rack it once again and then add potassium sorbate to prevent re-fermentation and finally add sugar to taste. This will maintain the current percent alcohol and allow you to adjust the sweetness.

My preference / prejudice is either option 1 or option 2 since I'm a big fan of adding minimal chemicals to the wine and I've blown up my share of bottles over the years with fermentations that restarted after sorbate. I know people who swear by sorbate / bi-sulphite so I list it as an option. It sounds like whatever you do, you've had a successful first batch - congratulations!