Does your 260 E have an auxilliary vacuum pump? I had a 280 that has one mounted on the front of the engine connected to the cam to drive it. Does your car have one of those? Does your engine smoke erratically?

Vacuum is the greatest from the engine at idle, so you are correct when accelerating you lose vacuum, that is why there is usually a resevoir or assited pump in the system. My 280 had an extra pump that helped with the brake booster. The first thing I would do is find the line to your brake booster, if you did your brakes you know where your fluid resevoir is, the booster is behind it, there you will see a line about 3/8 in size, trace that line. If it goes directly into the manifold make sure it is connected and not leaking. if it is ok, then start your engine and at idle, listen for any hissing noise, if you are losing that much vacuum you will hear it. If you can not find a leak, your booster may be failing, at that point hook up a vacuum pump to it and test it. Just pump it up and see if it hold a vacuum. If not it has probably failed and this is likely where you are losing vacuum.
If you have a vacuum assist pump make sure it has not failed by checking disconecting the line between the booster and the pump, hook a vacuum gauge on the nipple and start the car. So you could either have a bad pump, a broken vacuum line, or a bad booster. If the vacuum pump is failed it is usually the diaphram inside that only costs about $20
Let me know what you find out! Good luck!
Brian