It is a guide but never definitive.

Delays with the BA flight might mean VS makes it in earlier or vice versa. Possibly different routings that may cause flight lengths to differ. Also which airline has a gate that is closer to the immigration line.

It is always amusing to see if the BA or VS pax get to the immigration queues the first. Most holiday makers do not even they need to sprint when they look out of the window and say "oh look, the BA/VS plane is over there and it is moving as well".