MBobrik

As far as I understand it, due to the nature of vector addition, any thrust off your velocity direction is inherently less efficient. And the Oberth effect doesn't help you at all because it works through higher orbital velocity at the periapsis. And you haven't achieved orbital velocity yet during your ascent. . The most efficient way is always to apply thrust exactly in the direction of your prograde, so if you want to circularize efficiently, you should till apoapsis where your prograde is horizontal.