The term structure doesn't shift (1 for 1), it twists (1,.80,.65,.52....etc) between backwardation, flat, and contango. So you can be -1000 vega mo 2 and long 1550 vega mo 4 = net long 550 vol) but if vol moves, your p+l will probably be close to flat.
Now it can shift at times, but never on a...