I have made this in the past and it stunk the whole house out for days.
I suppose it depends what use you have planned for it, but if you just want to pickle some onions you can just add some onions, some pickling spices, some more onions and more pickling spices till your jar is full and then top up with ordinary malt vinegar. but you must first peel your onions and then layer them with alternate layers of salt and onions in a colander (spelling?) over a bowl overnight before washing drying and then pickling them and leave them to sit for about 3 months before you munch them.
but I am sure that you can buy already spiced vinegars.