If Vulcan orbited a gas giant, we probably ought to see this gas giant from space, too. That is, yes, sometimes the orbiting shots are framed tight enough that the gas giant might be behind the back of the cameraman (but then the gas giant should in turn be high up on the sky from the surface POV!), but we also get shots of starships approaching or departing Vulcan, and there's no companion planet there.
So the companion has to come and go, rather than stay. Which is the most welcome option in terms of general continuity anyway. Intriguingly, TAS "Yesteryear" shows a giant companion at the very same spot on the sky during day and subsequent night! Since Vulcan has a night-day cycle comparable to Earth's, as per that very episode, and since the only way for it to have that is to rotate around its own axis once a day, we either have to assume the companion orbits Vulcan with a one-day orbital period (which is problematic, and probably requires something to be constructed of styrofoam or neutronium) - or that Vulcan's companion was just visiting that night, speeding past the planet just right to hang on the sky above Shi'Kahr for a bit over half a day... Much like Remus would speed past Romulus (Remus aptly seems more distant from Romulus when Picard and Data make their escape than during the opening credits).
Timo Saloniemi