Vernon was advanced £45 10s. by the Exchequer in June 1416 to cover service in France, in which he was engaged for a few months. £40 a year was considered sufficient to support a knight, while £9 would pay an archer., so it is possible Vernon campaigned with a small force. If so, the most likely destination was Harfleur, where the English garrison was left stranded after the Agincourt campaign. The Armagnac besiegers were scattered in a daring relief operation, led by John of Lancaster, 1st Duke of Bedford on 15 August. This fits well with Vernon's subsequent career, as he must have returned to the Midlands by November.