I've always remembered/used the following:
Vx = lowest drag = best angle of climb at full power = best endurance at low power = lowest sink at no power
Vy = best L/D ratio = best rate of climb at full power = best range at low power = best glide at no power
This isn't entirely accurate because of prop/engine not being as efficient at low power/low speeds (therefore the "official" Vx and Vy speeds are slightly higher than the numbers you would get if you would only take the airframe into account), and it also does not incorporate wind drift. But as an approximation it works quite well.
Of course, if you plan a maximum endurance flight, or a maximum range flight, the numbers in the POH should be used instead. Or determine the numbers for your specific aircraft using the methods described earlier in this thread.