It's possible a tag was misapplied to the plant, or the original tomato had died and this volunteer/weed grew instead in its pot. In either case, this looks like a Morning Glory vine (in particular, probably Ivy-Leaved Morning Glory). They are notorious for being weedy from seed, spreading easily so they reappear for several years after first being planted, though the flowers are considered attractive as a decorative annual. If you don't want to keep it, either dig it out, or at least remove spent flowers (once it begins to bloom, which might be July or later) so it does not go to seed.

Miri