I agree with the prior responses. If the team has a slow start it will get ugly on Twitter, Reddit, and here -- and maybe even in the stadium (most likely if they lose a couple of early home games). We also will probably see some painfully awkward press conferences, at least until they stop making him available. But he gets a full season almost no matter what.
A slow start seems very likely if we don't have our 3rd DP until the summer.
In 2016 the club started terrible under Vieira. 18 points in the first 15 games. But it wasn't just a new coach and a new system and lots of turnover on the roster that caused it. Lampard played a total of 44 minutes in that stretch. He then played at least 75 minutes in each of the next 14 games, scored lots of goals off every body part, and the team earned 1.875 PPG to save the season. Jack Harrison had a similar injury/recovery schedule. Those 2 probably had more to do with the team's poor start and fast finish than anything Vieira did.
I would not be shocked if we have a similar trajectory to this season. People were pretty upset in 2016 (especially after RBW), but Vieira had the advantage of taking over after a bad season and began fresh in 2016, while the start of 2019 will be Dome's second half-season coming off a bad finish to 2019. If that happens it will get ugly, but the team stands by him.