The coarse nature of the VAS system makes it effectivey impossible to apply sensible modifiers. The lack of a maximum gunnery range also makes them difficult to employ sensibly within the confines of the rules. If you impose a maximum line of sight of 30"-32" you could allow observation aircraft to enable spoted gunnery against static targets above those distances out to the maximum range of the gun. However, to say that it wasn't playtested is a bit incongruous - perhaps not originally, but for OOB this aspect was debated at length and the conclusion reached was not that dissimilar from that reached historically, that their worth as far as aides to gunnery was concerned, was rather limited.