I cannot guarantee that they are safe. Some vegetables are able to be preserved with the vinegar and oil, some are not. It is oil that causes the problem.
I know these to be safe as they are from the University of Maine. I am going to post the link since there are other preserving recipes for them.
Plain and Pickled Fiddleheads
cider vinegar
sugar
1/8 teaspoon each of pepper, ground nutmeg, cinnamon,
allspice and celery seed

Pour enough vinegar over the fiddleheads to cover; then strain it off into a pan. Add 1 cup sugar for every gallon of vinegar. Add a large pinch of each of the spices and celery seed. Boil this syrup for 7-8 minutes; then pour over the fiddleheads in pint-sized jars. Seal and process for 10 minutes in a boiling water process canner.
Sweet Pickled Fiddleheads
1 quart cider vinegar
5 cups sugar
2 teaspoons salt

Mix vinegar, sugar and salt in saucepan; bring to a boil, pour over fiddleheads in pint-sized jars; seal; process 10 minutes in boiling water process canner. Makes 6 pints.

Here is a link that might be useful: University of Maine on preserving fiddleheads.

Thank you so much. I am going to call my MIL tomorrow. This is her recipe. I myself missplaced it.I will post it tomorrow. As much as I love my MIL she is old school.I just want to be safe. After I post the recipe please let me know what you think.TIA