Vidal was born in 1792, the youngest of four children of Emeric Vidal, who served in the Royal Navy. Alexander followed his father, embarking on a naval career in December 1803 when he joined HMS Illustrious as a 1st class volunteer. He served alternately under Sir Charles Hamilton, Michael Seymour and W. Shield. He initially served in the English Channel, off the north coast of Spain and in the West Indies, until November 1805.He later joined the Royal Naval College at Portsmouth, before joining Lavinia in November 1809. Here he served under Lord William Stuart, and George Digby. He spent three years aboard the Lavinia at the rank of midshipman, and saw service in the Mediterranean and West Indies, and at Cadiz and Lisbon.