I'm using Gran Compe non-spring assisted levers with Campy Veloce brakes

Can't seem to get the levers to retract back to neutral

Do the Campy brakes need the spring assisted levers?

Also regular cable housing ends aren't stopping it at the lever, should I be using a cable end button?

Mine Dont .. never have..

I expect you have excessive friction in the cable/housing run .. Get new Die Drawn Slicked surface cables ,& new Housing

take off the tape , create a smooth line from Brake lever to brake , no tight curves ..

Insure that it works Better, as You want, .. Then Retape the bars ..

Road Brake levers use a Pear shaped Ball , mountain, barrel shaped .

Do you still have the aluminum domed shaped DiaCompe cable ferrule ends?

Campy levers don't have a return assist spring, so you shouldn't need them with you Veloce brakes.

I suspect that your problem is either cable friction from a sharp bend or stiff grease (especially in winter), or related to the ferrule or lack in the lever. Housings need to end ar a clear dead stop. That means a ferule that bridges the hole in the lever properly, or a lever that has a built in fitting that ends in a flat bottomed hole with only a 2mm or so through hole.

BTW- at one time Dia Compe made levers that called for a threaded "ferrule". If you look under the lever and the hole is threaded you have to find the right part, or a stepped alternative that's supported squarely.

thanks guys,
I'll take the levers apart and inspect the ferrule or lack of one (also have some new cable out there with teflon inserts)

I think you mean this part?, that's what I was referring to in my OP as a cable end button (can't keep track of the right jargon)

IDK exact lever you got .. the unique one is domed to allow it to be fitting into the brake body at an angle
domed around the little hole . on the far end

that one is not it .. you show the ferrule for a non aero cable out the top brake lever ..

The inside the aero lever ferrule is similar but different then the one shown. The correct one is rounded on the small hole end, not having a stepped set of seating diameters.

I agree with Francis that cable friction is likely at play here. Brake calipers can have different return spring strengths. During the 1980s the spring strengths were often reduced and a lever spring was added. Not every brand did this at the same time and product managers (or aftermarket mechanics sometimes picked) a caliper with reduced spring strength and a lever w/o a helper spring. Then if the cables were not well picked/lubed/routed smoothly the weak caliper might not fully pull the lever to it's seat. (Very frustrating to have this condition).

Over the years we've replaced some older aero levers to ones with helper springs (and a lot of cable sets and bar wrap) when simple lubing and adjustments didn't do the job well enough. Andy.