BEK,
I have not seen the Oxford manuals to which you refer, but I can assure you that the sequence you quote is incorrect. For a prop VX is just less than VMP and VY is just greater. VMD is greater than all three. Best prop endurance is at VMP and best range at VMD. For a jet VX and best endurance are at VMD and VY and best range are slightly greater.
You should also be aware that the Perf exams often ask questions about how these speeds vary with things like mass and temperature, but they are shying away from the effects of increasing altitude (as discussed in other strings recently).
If you send me your e-mail address I will post you a whole aircraft L
polar diagram depicting these and many other (16 in all) interesting (for exam purposes) speeds.
If you look in the pilotwarehouse.co.uk web site under "JAR Question Banks" you will find an excellent book on the subject.