You would use the same amount of bottled lemon juice in relish recipes. Same for salsa. Be sure to use bottled lemon or lime juice since it has to meet a certain standard to be sold. Fresh lemons and limes vary in acidity and may not be acidic enough to preserve your food.
I have no idea how the flavor would compare, but it is not a safety issue. Bottled lemon juice is actually twice as acidic as 5 % vinegar, but the taste is more mild. I use bottled lemon juice in salsa recipes and it is good.
Perhaps just try a half batch of a relish or a jar or two of a quick pack pickle recipe and see how they taste.
Use a current recipe as many of the old recipes are not safe because years ago vinegar could be as much as 40 % acidity. You will need at least half bottled lemon juice to water ratio for a pickle brine.
If you try it, let me know how the taste is. I looked for a recipe for you but could not find one.
