I am not familiar with Bitter Sweet, but a quick google tells me that it is a deciduous vine (so it should normally loss its leaves in the fall). What else lost leaves? A true evergreen though (rhododendrons, holly, camelia, M. grandiflora or pine for that matter) should never completely defoliate, if it did, it's dead.

Evergreens do have any photosynthesis going on in the winter. Bitter Sweet are deciduous. One way to tell if a plant is dead is to dig your finger into the bark and peal it back and if you see green the plant is still alive

Well, peeling it back may be too extreme, but if you scratch gently at the bark, removing the top layer, the layer between the bark and the wood - the cambium layer - should be green if the branch is alive at that point. If it is brown/grey/wood color all the way to the wood, the branch is dead at that point. Try lower down in a few places, and see if there is green. If it's brown all the way to the ground, then the shrub is probably dead. HOWEVER, if the shrubs have been in the ground for a few months, don't give up hope until next spring, since if the root system is a nice strong one, shrubs MAY grow back from the roots. Suckering shrubs are especially good about this. If it's a grafted shrub, and it's dead to point of the graft, you might as well dig it out, as what may come back will be (less desirable) under-stock.

Have you been watering the plants? If so, how much and how often? What is the sun like, what is the soil like? How deeply did you plant, did you mulch, and if so, how deeply, how widely? How did the trees travel from the nursery to your house, who planted them, and how long had they been at your house (and how well watered) before they were planted? Knowing answers to these questions may help with advice.