I think the sliding top vents were sealed and the curtains removed in about 1980ish at least on the standard VEPs, as I seem to recall that Alan Williams wrote about it in his regular column in Modern Railways at about that time (the May 1980 issue IIRC). The Victoria-Gatwick 4-VEG units retained their curtains until they were converted back to standard VEPs in 1984 after the introduction of the Gatwick Express, though. Not sure if the VEGs also retained their opening top vents until then or had them sealed at the same time as the rest of the VEP fleet.
A few VEPs actually had the top vents removed and replaced by single pane windows, but this was never extended to the whole fleet.
In some ways I think it's a wonder that they bothered with those top vents in the first place, as all other units with a suburban style seating and door/window layout relied solely on the door droplights for ventilation, but I suppose as the VEPs were at least initially intended for long-distance semi-fast services such as Waterloo-Bournemouth it was probably thought that the sliding top vents would be less draughty (and thus more tolerable on longer-distance journeys) so passengers should be encouraged to open them rather than the door droplights.
I suspect that it was also only because they were meant for longer-distance services that they bothered with curtains so that they'd be on a par with the CEPs, CIGs, REPs, TCs and the like, all of which had curtains throughout when built.
