What preservative should be used for pickling vegetables like carrot, cauliflower and green chillies which are parboiled and could be used without refrigeration?
Pickling is in itself a preservation method, based on the acidity of the pickle.
There are two ways to achieve this:
If you want to keep pickled vegetables without refrigeration, you should use only trusted recipes and methods which are from highly respectable sources, as improperly pickled and preserved foods can be dangerous.
Note that there are many recipes for "refrigerator pickles" which have a shelf life of several weeks, but are not intended for long term preservation. Make sure that the recipe you choose is intended for long term storage. These almost always involve either true fermentation, or boiling water or pressure canning.