An aircraft, or more correctly the wing, does not necessarily stall when it loses speed. It stalls when the wing exceeds the stalling angle of attack. In straight and level unaccelerated flight, ie at 1 g, that angle is proportional to Indicated Air Speed and is called Vs. In a 60 degree bank level turn, you would be pulling 2g. In that case the stalling speed would be Straight and Level Vs√g = Vs x 1.44. In a vertical climb the g would be zero, so the stalling speed would be zero knots, which explains why you can still roll in the vertical when below the Straight and Level stalling speed. Get your insrtuctor to give you a few hours in a Cap 10 or a similar aerobatic aircraft and it will all fall into place. It's a lot of fun too!
