Another two balloon idea
I haven't completely finished thinking this idea through yet, but anyway...
The basis of the idea is to use one balloon inside the other, both being partially inflated but so that the sum of their inflations equals that of a single balloon. The release is then triggered by the bursting of the outer balloon, which will be larger and therefore under more stress than the inner balloon. The inner balloon, being under less stress, should not burst and whilst having insufficient lift to maintain altitude, should result in a controlled and reduced rate of descent, at which point Vulture 2 is sent on its way.
Fabricate a short length of coaxial tubing with, at the upper balloon attachment end, the inner length of tubing being longer than the outer. The two coaxial tubes need to be brought to separate inflation feeds at the bottom inflation end of the combined coaxial tube. With both balloons deflated, attach one balloon to the inner coaxial tube and then carefully feed this balloon inside the other, which is then attached to the outer coaxial tube.
When the balloons are to be inflated, partially inflate the outer balloon first and then inflate the inner balloon, the aim being to achieve the same total volume and pressures in the combined balloons as you would in a single balloon.
The thinking behind this is that you start by imagining a single inflated balloon and then ask what would happen if there was an internal membrane separating the inner volume of the balloon from its outer volume? The pressure within the balloon was uniform within the total volume before introducing the membrane and just introducing the membrane should not change this, so the pressures inside and outside the membrane will still be equal. Where it gets more tricky is when you try to factor in the stresses on the envelopes of the two balloons when both are under positive but equal pressure and how the relationship between the pressure of the two balloons will change as they ascend and expand.
I can see two potential issues with this straight away: the risk of the shock of the outer balloon bursting triggering the inner balloon to burst as well, and detecting the bursting of the outer balloon.
I'm envisioning the two balloons being inflated to somewhere between 50:50 to 75:25 percent (inner:outer) so that there would be quite some clearance between the two balloon envelopes, which should increase in absolute terms as both balloons expand and which, I would hope, would provide sufficient safety margin against the shock of the outer balloon bursting triggering the burst of the inner.
As to detecting the bursting of the outer balloon, the best way I can think of is by some sort of shock gauge, although I suspect that this would need to be disarmed until the whole ensemble has reached relatively smooth high-altitude air, to avoid being triggered by low-altitude turbulence.