gwl, just my 2 cents, but I think that any sort of warning is a slippery slope. By doing this you're basically playing Simon Says with the dog. Once a pup understands a command hey should obey, if they don't obey they get discipline. A warning vibrate adds a step and teaches them they only have to obey if they get a command AND get warned.
I run the beep=whistle, one for eye contact two for recall and I'm adding the vibrate as remote down.
Some folks I know use a trill whistle to remote down their dogs, both as a safety feature and as a timeout of sorts. You could try that, so if a dog is creeping on point you down it - a punishment by denial without electricity. But then you'll also have the added safety feature of silent remote down if they run towards a road or porcupine. I really like using this when we are pheasant hunting a discover a pond. You can down the dog at distance to try and jump shoot ducks without yell/whistle.