The Baggies are keen on Johnstone, and see him as a possible successor for understudy Boaz Myhill, whose contract is up in the summer.
And the 24-year-old, who is currently on a season-long loan at Villa Park from Manchester United, has told close friends he is heading to The Hawthorns.
But Albion boss Alan Pardew is far more concerned with signing a striker and an attacking midfielder in January and reports of a £4m fee being agreed with United are premature.
Johnstone has played every single minute in the Championship for Villa so far, and boss Steve Bruce is keen to keep him until the end of the campaign.
But there is a recall option included in his deal making a January move possible.
However, the Baggies have only scored two goals in Pardew's first six games in charge, and they are planning to focus all their available funds to the other end of the pitch.
Albion's No.1 Ben Foster was supporters' player of the season and players' player of the season in the last campaign and goalkeeping is not an area in need of immediate help.