Born in Chertsey, Surrey, Vellacott qualifies for Scotland through his mother, who is from Peterhead. He came through Hartpury College and represented Scotland at U17, U18 and U20 level, scoring on his U20 debut against Ireland and featuring at the Junior World Championship in 2015.

His professional breakthrough came at Gloucester in 2017/18, when he made 27 appearances and scored seven tries, earning a call-up to train with the England squad.

He scored 14 tries in 49 appearances at Kingsholm before joining Wasps in 2019, though a torn ACL soon after his arrival ruled him out for much of his first season.

Vellacott moved north to Edinburgh ahead of the 2021/22 season and made an instant impression.

He was named Player of the Match on his debut in the home win over Scarlets, then again a week later in Treviso, where he scored his first two tries for the club. That form earned his first Scotland cap off the bench in the 33-22 Six Nations win over Italy in Rome in March 2022.

A respected figure in the dressing room, Vellacott was named co-captain for the 2023/24 season.

He brought up his 50th Edinburgh appearance with a big away win over Scarlets in the EPCR Challenge Cup, and went on to lead the side as club captain in 2024/25, the season Edinburgh returned to the URC play-offs.