When Vines grow over the face of an Observer, the Observer doesn't output a redstone signal. When the Vines are added manually or removed by any means, it does output a redstone signal. It's just Vines growth that doesn't produce a redstone signal.

To reproduce:

1) Place an Observer facing in one of the cardinal directions.

2) Place redstone dust from the back of the Observer around to the front where you can see whether it's given a signal or not.

3) Place a solid block above the Observer.

4) Place Vines on the solid block above the Observer, on the Observer-face side of the solid block.

5) Increase randomtickspeed so the Vines grow faster. I set it to 100 and it happened quickly enough.

6) When the Vines grow over the face of the Observer, the redstone dust doesn't light up. When the Vines on the Observer's face are broken, the redstone dust doesn't light up. The redstone dust also lights up if the Vines are broken by any means.