thanks, betsy, for your replies.  after some investigation, i am pretty sure what i have here is small box elders.  it makes sense as we had an old box elder in our yard which came crashing down a couple years ago.  'don't know how i didn't notice these trees until this summer, but...  we are still stumped by the vine/tree we managed to kill last summer.  i took a good look at it again and, sure enough, the branches 15-20 feet up were definitly growing off the massive vine.  we haven't even thought about how we might take this vine down now that it is dead.  i wish i'd taken photos before we sprayed and cut the vine.