You are totally entitled to a refund of the cancelled leg. It shouldn't screw up your return flight. (note to your future self, there's no advantage to booking return flights with VY, and several advantages to booking one-ways...) However, is your return flight also VLC-LGW? I can't see any other VY flights so they might have dropped the route.

EC261 / UK261 does make fairly clear that VY should rebook you on another airline. I'd make a token effort to convince VY to do so, but they probably won't. https://europa.eu/youreurope/citizen...cancellation-1

Under the assumption that you are going to rebook yourself on Easyjet anyhow, just go ahead and book that, then take VY to MCOL to reimburse you the difference between the Easyjet fare and the one originally paid to Vueling. (and presumably refunded). [edit: it looks like Easyjet flights are only £20+ anyhow, so I wouldn't stress too much - take the VY refund and book Easyjet]