It is not a rumour or a possibility. The 2026 Vuelta a Espana will start in the microstate of Monaco in 2026 and that has been officially confirmed by race organizers.
The small city, in the French Riviera, is located right by the city of Nice which will host this year's Tour de France finale and will host the start of the 2026 Vuelta. Monaco has, in the past, already hosted the Grand Depart of the Tour de France back in 2009 with a time-trial won by Fabian Cancellara.