Q. Can I salt a bowl of fresh vegetables on Shabbos right before a meal? (Cucumbers, peppers, etc,). Does it make a difference if I add some vinegar to it? or oil? (which I prefer not to use,)

A. The Talmud (Shabbos 108b). teaches that it is forbidden to salt radishes on Shabbos. There is a debate among the Rishonim, as to what is the exact nature of the prohibition. Rambam (Shabbos 22:10) and Shulchan Aruch (321: 3) maintain that the reason is because it appears like marinating things on Shabbos. Marinating is prohibited because it is similar to cooking, which is a main forbidden melacha on Shabbos. This is forbidden Rabbinically. (Mishna Berura 321: 14)
Rashi, however maintains, that the proscription is because the salt hardens and preserves the radishes from spoiling. This would be akin to the prohibited melacha of meabeid or tanning, as in preserving hides needed in the building of the Mishkan. Since the Talmud (Shabbos 75b) rules that meaved does not Biblically apply to foods, the Mishna Berura (321:15) writes that this is only a Rabbinic prohibition. However, it is not limited only to radishes, and it extends to other vegetables too, when it is customary to preserve them by salting.
Mishna Berura (ibid.) and most Poskim permit salting individual pieces of vegetables when one eats them immediately. Salting many is also permitted when vinegar or oil is poured immediately after. The reason is that there is no appearance of ivud or that the added liquids diminish the preserving power of the salt.
Foods and vegetables that are usually not preserved by salting, such as eggs or tomatoes can be salted when they will be eaten on that day. Preferably they should be salted right before the meal (ibid. 21)
Shmiras Shabbos K’Hilchaso (11: n6) quoting Horav S. Z. Auerbach zt”l maintains that although tomatoes are indeed pickled, they are only pickled whole and not in slices and thus adding salt to slices does not resemble pickling.
Adding vinegar, oil or dressings to salads is permitted, since it is not usually done for preserving purposes.
Horav Shlomo Miller’s Shlit”a opinion is that adding vinegar should be restricted to foods that are not usually preserved with vinegar, or by using small quantities, making it obvious that it is only added for flavor. (See Piskei Teshuvos ibid.)

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit”a