The magnetic vortex formation at room temperature and its evolution under in-plane magnetic field is studied in chemically grown self-assembled La_0.7Sr_0.3MnO₃ nanoislands of less than 200 nm in width. We use variable field magnetic force microscopy and numerical simulations to confirm that the vortex state is ubiquitous in these square-base pyramid shape epitaxial La_0.7Sr_0.3MnO₃ nanostructures, and that it requires in-plane magnetic fields below 40 kA/m to be annihilated.