I just freeze the leftovers of my tomato paste in the can. Once it's solid, remove the bottom, push
out the paste like a tomato-paste push-up, put it in a zip-lock and store it in the freezer. Next time you need it, pull it out, slice off however much you need then throw it back in the freezer 'til the next time. Easy, Peasy.
And by the way, I think Julia would roll over in her grave if she knew you were adulterating that recipe by not using reduced vinegar but if that's okay with you . . . what next, Boyardee instead of the sauce? (Je suis si decu.)