Sean,
I hope like hell you are not teaching or thinking "a safe altitude to turn back in a C172 is xxxx feet".
If you are trying this out in flight then you are planning for it without including startle factor, reaction time etc so the testing you are doing to find out that altitude is not valid for a real world engine failure. Loss of about 200 feet prior to putting a plan into action is about what I see with my students when they are not expecting an EFATO.
There are far to many variables to make bold statements. What works at a sea level airfield will not work at a 3000 AMSL airfield or an airfield with 3000 feet PA or DA. A MAUW C172M would be lucky to do 300 fpm at a 3000 foot PA airport and that will put you a long way up wind before your magic altitude.
Trying to land back at a 2000 meter long will give you a false sense of confidence. What about landing back on a 400 meter airport?
What will the ground speed be on the attempt to return? What would be your limit? What radius of turn do you need to return to the runway? What is the aircrafts limit? How much energy will you have to dissapate before you go through an immovable object (runway end fence for example) and survive?
What about the aircraft taking off just behind you, how big is it and can it move quickly out of your way, how skilled is that pilot, and does he see what you are up to?
Is there a steep wind gradient on takeoff, or sink at the departure end?
A MTOW C172 will probably not make it to the airfield (nil wind) from the altitudes you are quoting. (If you insist on being a test pilot and to try it please do not load Pax to increase the weight).
Back to your question Vx or Vy.
Answer - It depends on the conditions on the day and the airport you are flying from.
When you look at all the variables the safe way is to land straight ahead into wind or there abouts, and hit the softest thing you can find (aircraft are easy to replace). Yes at some point a return to the runway may be possible but are you willing to BET YOUR LIFE and YOUR PASSENGERS LIFE that you have the skill and the luck to make it?
Finally thanks for bringing up the topic, great discussion so far.