Never set foot in a Harrier but got to test guns tracking vs VIFF. As I recall the VIFF required only needed to be very brief to disturb a close-in pipper chase with a shorter-range gun (eg Aden 30) - but, as said, you are still giving away some energy and timing was everything. When the opponent had a somewhat better radar and gun combination (eg Mauser 27mm) the increased distance at typical 'open' range made the effect of the vectoring pretty much unnoticeable, at least from my perspective. About the only noticeable improvement during this DACT for the Harrier's perspective was that it was much harder to spot or maintain tally when it is flown with clean wings. Or it could be just my eyes...
A more dramatic change in a guns picture could be achieved by the venerable mud-moving Tornado. With the sight picture looking otherwise rosey it can be amazing how quickly a turning GR1 can slow down when the wings are suddenly pulled all the way back. I also remember a USAF Spangdahlem F-16 dude managing to over-stress his stores whilst trying to get out of the way of the GR1 he had just chased down when it did the wings & boards thing. Given the rather pedestrian G envelope of the Tornado that didn't go down too well and the bubble-bust did look a little worrying on the HUD tape.