I dehydrate vegetables and keep them in glass jars for a long time. Is it a good idea to put a small amount of alcohol 70% in the jar before I insert the vegetables, in order to kill the bacterias? Is it a good idea to put some salt inside? Does it prolong the shelf life of the dehydrated vegetables? Does it have any side-effects (besides a small effect on taste)?
I don't understand what you are trying to achieve here.
Food, or at least vegetables, is spoiled by bacteria (sometimes also mould). Bacteria need quite a few factors to maintain homeostasis and live. They can't live if 1) a toxin is present, or 2) their living conditions are not met.
When you preserve food, you remove one of the conditions bacteria need to live. In the case of dehydrated vegetables, you remove the water needed for all the biological processes in their cells. From now on, each bacteria which falls onto your vegetable will die without multiplying. You will never get a bacterial colony on your dehydrated vegetables.
Pickling vegetables is another preservation method. You can use for example a salty brine. At sufficiently high levels, salt is a toxin for every organism. If you want pickled vegetables, you can put them in a brine with sufficient salt levels and keep them there. Again, any bacteria which fall in there will die.
Using both at once on the same vegetable is useless. You have no bacteria living on it any more - you don't need a second way to kill them. Putting salt on dehydrated vegetables is like putting rat poison on the moon - it doesn't reduce the rat population, because it started out with zero rats.
Food safety wise, dehydrated vegetables will last forever (and if they don't, you didn't dehydrate them properly) - so why would you want to extend their shelf life?
If you don't want to eat ancient dehydrated vegetables, this is probably because of their taste. They are not unsafe, the aromatics in them have decomposed with time. This is also a kind of "shelf life" - but it gets shortened, not prolonged, by adding prodigious amounts of salt, alcohol, sugar or other preservatives. With them, your vegetables taste worse from the start.
I never heard/read that using alcohol would help?! I've seen it used mostly for fruits.
Salt is used (and has been used) to keep meat and fish for a long time , but I never heard about using it for vegetables; Me think it would not really change anything on the long run; but when using the dried vegetables for a recipe, you will need to be extra careful about the extra salt (either do not add more, or rinse in fresh water before using it.
A quick google suggest that keeping dried food stuff in sealed containers in cool and dark places will be enough for a long shelflife.