The Brazilian winger has been in outstanding form for the LaLiga leaders since the season's restart, but his involvement in their next match was in doubt.
Madrid head coach Zinedine Zidane revealed on Thursday that Vinicius had missed training to take a coronavirus test.
An initial test "went badly" - seemingly returning an anomalous result - and had to be repeated, Zidane said.
Madrid offered no update on Vinicius but named the 19-year-old among their 22-man group for the meeting with Alaves.
Zidane's side are a point ahead of Barcelona, who have played a game more.