VS will try to seat family groups together (as do BA) but they have to be on the same booking I believe. This makes the whole thing easier for all concerned, saving any hassle on date of travel. They cannot guarantee this but they will do their best.
Of course, if there is a problem, you are at the mercy of the check-in staff and the willingness of them to actually provide some sort of customer service. This will invariably be where "the wheels fall off" and all the trouble starts!!