A/P Basics
Remember the following relation between Autopilot and Autothrust:
A/P and A/T always WORK TOGETHER:
When changing altitude, A/T supplies a given amount of thrust, A/P controls pitch to maintain desired speed.
When maintaining altitude, A/P controls pitch to maintain altitude, A/T controls thrust to maintain desired speed.
(Acquisition of altitude, in this regard, is conceptually the same as maintaining altitude).
Vertical speed mode is a strange mode in this respect: A/P controls pitch to maintain desired rate of climb or descent, A/T controls thrust to maintain desired speed.
HOWEVER, if the rate of altitude change is chosen unwisely, speed control will suffer.
If the chosen rate of climb is too high for the amount of available (CLB) thrust, speed will drop below desired value, eventually all the way to the various low speed protection that may be built into the A/P A/T system.
If the chosen rate of descent is too great for the minimum amount of (idle) thrust, speed will increase above desired value, eventually all the way into the various overspeed warnings and/or protections that may be built into the A/P A/T system.
A 'gotcha' of this mode is the decrease of climb thrust with altitude: a rate of climb that can be maintained at 10.000 feet, may be too high a rate at 25.000 ft, so pilot attention is required when using V/S mode.
If you look at scenario's of A/P engaged, but thrust controlled manually and even (deliberately) mishandled, yes, than things may get out of control. If you want to figure out 'how much out of control?', then reason along the rules I have stated above.
By the way, (Boeing) VNAV or (Airbus) CLB work along the lines described above for altitude change, the only thing is that the FMC / FMGS will automatically change desired speed, according to the programmed rules (e.g. 250 kts below 10.000 ft).
In VNAV PATH descent, respectively managed DEScent, the A/P in principle controls pitch to follow the planned PATH, while A/T controls thrust to maintain speed. The difference with V/S mode lies in the fact that the guidance system will have set limits within which the speed is allowed to vary - if the speed gets too high, even at idle thrust, the pitch mode will revert to SPEED mode, controlling pitch to keep speed at a safe value. Of course, in a descent, thrust should never be insufficient to maintain speed.
Hope this makes matters clear.
Happy gliding. Do you really still fly a Ka-8? (beautiful aircraft it still is).