heh, Ed, my opinion on weighted vest use is the exact oposite to yours, in that using the extra resistance for calisthenic and plyometric exercises is beneficial, whilst wearing it for bag work/kata would actually be detrimental as you are introducing an unnatural weight distribution to your techniques, potentialy diminishing your bodies ability to perform the movements with only its natural weight and balance.
To make the core work harder, the weight involved needs to be held/moved from away from the body. Dynamic core work with medicine ball and sledge hammer is better than merely adding a gravitational force to the spine.
Tim, weighted push ups are a very different animal to bench press, involving a lot more core engagment, and differing strain on the antagonistic muscles due to a different range of movement.
As they are different, they can both be considered equally as valid.
As for holding a dumbell between your feet for dips and chins, thats ok too, but can be tricky with a heavy weight, and can lead to concentrating on getting that right and not the actual exercise you are holding it for.
Horses for courses, as they say