That yellow vector appears because you are trying to do the impossible with that WWI model with the route specified.
That yellow vector represents what the model will actually do when trying to follow that route, if that segment was possible, then it would blend in with the usual way-points.
When laying out your way-points, think about "can this aircraft do this in the amount of time I give it", even if you are not sure, don't do it, it can cause serious side-effects "like crashing" when attempting to run the simulator. This is especially true for bombers.
From what I saw, you are trying to encourage a fight. Remember, once the two aircraft meet and engage each other, the route is semi-ignored. The route will only act like a timer until the enemy is dead, from there it will head to where it should be along your route at that specific time.
When making a mission that contains a fight, make it as simple as possible, it may not be pretty to look at but it works a lot better.