Good post C100driver, thank you.
I agree mostly with everything you say. But do not get me wrong, if you re-read my posts in this thread you will see that I have never mentioned any specific altitude, and I have always said that it varies with atmospheric conditions etc. etc..
This is definitely a maneuver one must train for to be able to accomplish, and one must of course be aware of all the factors you mention to make the safe decision of turning back to the runway if that is possible. This is always also why I was taught to always do an emergency briefing prior to departure to go through all this before hand mentally and decide whether conditions would favor turning back or not etc. etc...In the end as I have mentioned countless times it's all about knowing what you and your airplane are capable of doing. If ever any doubt, landing straight ahead is the best option. However, as I once again already have said, if I know for sure that I can make it (partially because I have done it before), I won't hesitate.
Good training pays off, and one day it might save my life or save me from injury.
Back to your question Vx or Vy.
Answer - It depends on the conditions on the day and the airport you are flying from.
I am curious to know which conditions you think will favor Vx over Vy in this situation, if you do not mind sharing ?