So if they managed to take Notthern Gaul and settled there first before being pushed across the Channel?
Giving the position of Vandals, if they were to definitely settle in Gaul, I would see central and/or southern Gaul being more obvious targets (more or less taking on what Burgundian or Goths did IOTL) for them being richer.
Not that they didn't raided and campaigned in Northern Gaul IOTL,
but even defeating Franks, they seem to simply not have been interested on the region for establishment purpose.
Or if the Romans used them as mercenaries in Britain?
Well, it actually may have been the case historically : Romans hired mercenaries of various origins to fight in their provinces, including Brittania.
But they were that, mercenaries, warriors, not a people or settlers.
Situation is a bit different with
laeti, but that implied usually a dominee and isolated position for the concerned peoples, in relativly few numbers.
Establishing
laeti in Britain wouldn't be really far-fetched if you manage to increase the Barbarian pressure on it during the IIIrd and IVth centuries, but that alone wouldn't lead to a real regional settlement.
Dediticii Vandals wouldn't be too much of a stretch but again, their inferior position (defeated peoples, more or less "public slaves") doesn't fare well for possibilities of takeover.
It would work better with
foederati, but there Vandals are relativly bad candidate, as this status was often an aposteriori agreement (or at best, negociation with a warring people) and Vandals had no real reason to go for Britain when they could go for Spain or Africa or maybe Gaul, Italy or even Pannonia, all better prospect than Britain.
Even if you had an important Vandalic presence in Britain enough (trough any of the aformentioned possibilities, or probably all of them, except foedus), I don't think it would be strategically and/or numerically relevent enough to fend off Saxons and Western Germans (a bit like
Alan foedi or Saxons in Gaul didn't against Franks)
Or would that just be implausible
I think that it may be so, while a more or less limited Vandalic presence in Britain isn't.
It would imply Vandals being strong enough to defeat Franks, but weak enough to be prevented to advance further south, strong enough to take a fleet and defeat Romans in Britain, but weak enough to loose northern Gaul and eventually strong enough to defeat Saxons.
It's not impossible, but would require a chain of contradicting events, in my opinion.