This has been discussed many times, including
this thread, which I think is the most recent one.
Simply put, almost every plane in AH except the Spits, N1K2, Hurricanes, Zeros and the P-38L.. needs to save a considerable margin of speed prior to the reversal while vertical, or it will fall under the dreaded flat spin. Which means, around 100mph, you have to start heading your nose down.
Usually this tendency is a lot higher on combat trim than manual, but in some planes like the F4U-1 series, it is dangerous to try a pure vertical even with manual.
..
It is also one of the reasons why friggin' N1K2s and Spit9s can snipe you out of your vertical - they don't need to save the '100mph' like other planes. Just nose up straight, aim and shoot until your plane absolutely cannot nose-up any longer(which is around 30~40mph), and then ease a bit on the controls and gain some speed, and they recover control of the plane almost instantly. When you have to go through the reversal, and start pointing your plane nose to the ground at about 70 mph, the Spit9 and the N1K2 is still pointing the guns at you..

but that's for another thread...