When you find cheap lemons, buy a lot.
Squeeze half a lemon in each of the cavities of an ice tray.
Freeze.
Within a day, remove the frozen slivers from the tray to a ziploc bag in the freezer.
You now have measured units of fresh lemon juice you may use for cooking and will keep for months.
The frozen lemons are a bit less acid than fresh juice, but full of flavor.
You can do the same with limes.
cooking.stackexchange.com/users/183/papin
Based on this site you can substitute the lemon juice for either an equal amounts of lime juice, an equal amount of white wine or half the required lemon amount of mild vinegar (like you mentioned)
cooking.stackexchange.com/users/27/Kyra
If you're doing it for the acid (i.e. to cut the heat in a spicy dish), you might try cream of tartar if you have it lying around.
I've never actually tried to substitute tartar for lemon juice, but lemon juice is the most common substitution for cream of tartar, so it stands to reason that it works both ways.
(Note: You would use about 1/3 as much cream of tartar as the amount of lemon juice that's called for).
If it's for general flavouring, this may sound insane, but if you happen to have any cherry brandy or even regular brandy lying around, try that.
A solution of sherry and cider vinegar is an OK substitute but doesn't quite have the tartness and strength of lemon juice.
While cherry brandy obviously doesn't taste the same as lemon juice, it's often just as good or better in recipes that call for it.
cooking.stackexchange.com/users/41/Aaronut
if your using the lemon juice for the acidic aspect then you can use 1/2 as much vinegar.
However if it for flavoring I would substitute another juice such as lime or orange.
Sometimes you can also you lemon extract for flavoring.
cooking.stackexchange.com/users/83/Zaphoid
Whenever I have to substitute something I have to remember I am no longer making the same thing as what was in the recipe or that I had started with.
With that in mind I am more free to create then to agonize over trying to recreate.
I have had some great success with this, and some that should best be left in the past :)
As a substitute for lemon, I think I would try a different direction, rather then try and recreate the lemon, go with salty.
Like a soy sauce, and perhaps simmer some apples in the soy sauce, or reduce apples in a little water then add soy.
You have done something similar to the lemon but yet entirely different.
Recreation is very hard, and you are always left with the unmet expectation of what should or could have been.
cooking.stackexchange.com/users/41906/N Scott McPherson
In the Middle East, a common substitute for lemon juice in salad dressings, hummus or other savory dishes is citric acid.
1/4 teaspoon citric acid equals 1 tablespoon lemon juice.
cooking.stackexchange.com/users/72455/Becky
Two UNreasonable substitutions are alum (toxic in large doses) and citric acid.
cooking.stackexchange.com/users/649/Tim Gilbert
Most of the recipes where you need to add acid, it's about the acid.
Either balancing out the salt, or the flavor, or to help chemical reactions.
Lemon juice and vinegar are different acids, but if you are from countries where lemons are not the fruit of origin, use what is in your country.
Nice apple cider vinegar and use as substitute mentioned above.
Vinegar has two forms.
Both are present in your bottle of vinegar.
One is liquid and one is very volatile and needs to be cooked off
(that means using vinegar in cooking
you want to cook off the vapors a bit.
So in cold dishes, before adding vinegar is good to heat it up for 20 minutes if needed).
cooking.stackexchange.com/users/11540/skriatok
I just made some humus and half way through, realized I had no Lemons.
ARRRRRGH!!!! I did have some rice wine vinegar and some frozen Orange Juice.
So I went with that.
But now, thinking about it a bit, maybe it should have been rice wine vinegar an worcestershire sauce.
Lemons are Sour, Acidic and bitter.
A bit of bitter might have been better.
Note: I did not use the Pinot Noir.
I drank that.
Hence this silly comment.
cooking.stackexchange.com/users/20823/Gustave
In the past I cooked myself into a corner when I realized mid-recipe that I didn't have any lemons or lemon juice available. Nor did I have any limes.   I can't remember the exact recipe, but I believe it was some baked fish dish. What are some possible substitutes for lemon juice in this application? I ended up using a dash of apple cider vinegar. It didn't turn out bad, it just was distinctly  not  lemon.
Are there any reasonable substitutions for lemon juice?
cooking.stackexchange.com
<substitutions><lemon-juice>
cooking.stackexchange.com/questions/1045
cooking.stackexchange.com/users/60/hobodave
The best solution would depend on the application, but you can prepare ahead by freezing lemon juice in an ice tray. Citric acid, lime juice, white wine, brandy, vinegar or cream of tartar can add acidity, or salty flavours can make the absence less noticeable. Other acidic products such as vinegar, white wine, citric acid or cream of tartar will replace lemon juice. You can also try cherry brandy for a slightly different and improved flavor.
You can stock up on lemons and freez the juice in ice cube trays to always have some in stock. Alternatively, keep some lemon extract or cream or tartar in your stocks. Try substituting with cherry brandy, half the amount or vinegar or white wine. Try a different flavor such as salty instead of acidic.