Vandoorne & Gutierrez are not contracted reserves for tRacing Point or McLaren (that's from next season on, no?), but Mercedes allows both teams to choose from Mercedes' reserve pool if they need and want to.
Racing Point also sharing Mercedes reserve drivers with McLaren
Mercedes has made arrangements to share its reserve drivers with McLaren and Racing Point if they are required during the 2020 F1 season.
www.racefans.net
"Mercedes has made arrangements to share its reserve drivers with two other teams if they are required during the 2020 F1 season.
As reported earlier, Mercedes’ future engine customer McLaren will be able to call on Stoffel Vandoorne or Esteban Gutierrez if a substitute is needed for Carlos Sainz Jnr or Lando Norris this year.
RaceFans understands Racing Point, a long-term Mercedes engine customer, also has the same arrangement in place.
Vandoorne and Gutierrez could therefore find themselves placed in one of three different cars if they are needed this year."
I would assume "arrangements" would involve some sort of contract. Considering Racing Point and McLaren have no other named reserve drivers, I would think that would make the two contracted reserves for all three teams. Perhaps there is some legalese in the F1 regulations that state otherwise, but that is well beyond my depth of knowledge on the subject.
Regardless, Russell made the most sense for Mercedes to take the seat this weekend, just as Hulkenberg made the most sense for Racing Point. It certainly adds something interesting to watch this weekend.