[3] In 2012, he was included in the Free State's Under-18 Craven Week squad, scoring a try against KwaZulu-Natal[4] and he played in the competition again in 2013, this time contributing a brace in their match against the Pumas.
[8] A knee injury ruled him out for part of the season,[9] but he returned to action for the Western Province U21 side during the 2014 Under-21 Provincial Championship, scoring five tries in five appearances, including a brace against the Leopards U21.
[11] On 2 July 2019, English Premiership Rugby side Leicester Tigers announced the signing of Viljoen ahead of the 2019–20 season.
[13] In March 2015, Viljoen was named in an extended South Africa Under-20 training group as part of their preparation for the 2015 World Rugby Under 20 Championship.
[14] He didn't feature in a friendly match against a Varsity Cup Dream Team[15] or their two-match tour of Argentina,[16] but was included in the final squad for the 2015 World Rugby Under 20 Championship.