In a sense, all thermals are vortex rings. Even though a a plume thermal appears to consist entirely of a rising column of air entirely sustained by in rushing, warm, surface, air, there is more to the system. A plume thermal is also surrounded by an large area of cooler air gently settling to replace the warm air rushing upward. A plume thermal can be considered a vortex ring attached to the ground. However, the turbulent, chaotic, convection systems do not necessarily follow the rules, and often organize in surprising ways.
Monarch butterflies are often able to exploit thermals near the ground.
The second diagram on the right indicates how migrating monarch butterflies fly upward to enter a plume thermal from the bottom. Butterflies initially proceeding by cruising flight within 3 meters of the ground will abruptly begin to fly upward, beating their wings more frequently, as they gain altitude. It is not know how the butterflies are able to recognize that they are at the base of a thermal. On a typical day under the influence of a high pressure system the morning begins with a clear sky, the ground feels cool to the touch, and there is no wind. The nocturnal inversion is in place with cooler, denser air at the surface.