Martin Gregorie wrote:
...
Kindly draw the vector diagram before continuing. You'll see that the
sinking speed velocity vector points down and the rising air vector
points up. Simple vector addition says that the rising air velocity is
subtracted from the sinking speed because their directions are
opposite.
...

But this is near like adding apples an oranges. The vector you are
interested in is the relative wind vector, i.e. the velocity vector
of the airmass seen from the aircraft as reference frame, for what
concerns the AOA. Before entering lift, you can consider it as the
sum of 2 components, a horizontal one, opposite of the horizontal speed
of the glider, and a vertical one, opposite of the sinking speed.
When lift is entered, a 3rd componemt is added, the lift vector. This
3rd component has the same direction as the 2nd one, i.e. both are
upward. So the new relative wind clearly causes a higher AOA. Adding
the sinking speed vector and the rising air vector, while mathematically
possible, has no physical sense.