The CIGs came in two batches, first about 1963-64 to replace the 1930s Sussex Electrification stock, were an update of the Kent coast CEPs. Main difference was a tidier front end and motors under a central coach, instead of motors at outer ends. The guards compartment in motor coach was therefore in middle instead of smaller ones behind each cab.
Second batches were built 1966-1974, and the batches of VEPs and CIGs were built same years. The later CIGs took over from the 1937-38 4-COR units. Some of the earlier VEPs were all blue rather than mainline blue+grey, but later on the all blue was restricted to suburban stock.
The VEPs were for stopping trains so had doors at every seating bay, the CEPs only had doors at end and a middle door. The guards van was bigger (with 2 double doors on a VEP). The location of first class was also swapped, on CIGs was on outer (cab) end, on VEPs was on other end, with toilet by gangway connection. Both had 4 compartments but on CEPs one was an 8 seat second (but those in the know knew it had more legroom as it was first class size).
The CIGs composite vehicles had an odd row of 4 seats (by middle door), otherwise were 2+2 seating. I think these had tables originally but they were detachable and I only remember seeing them with small drinks tables below the windows. VEPs had 3+2 seating and because of the doors the luggage racks were V shaped metal racks facing sideways above the seat backs.
I remember VEPs with orange curtains, but I think they used to get dragged into the doorways by disembarking passengers, so didn’t last very long, but the CIGs kept them longer. In later years another first class compartment got downgraded, and the VEPs had part of luggage van converted to 2 extra seating bays of seats (but both rows didn’t all get doors). The lighting and saloon panelling also got changed in later life, and public address was added
There was a buffet equivalent of CIGs (BIGs) which simply substituted the trailer second for a buffet car
By Network SouthEast era, some buffet cars had been replaced by converted hauled stock, and the VEPs were used on stopping trains and busy peak hour trains. However in later years wasn’t uncommon to see mixed formations.