Yup, a lot of people are confused because they think the 200% means "it adds 200%, so you get x3", when instead it is "you get 200% of the base xp, so it is x2". Also, the effects do stack, but they do not multiply each other, so two times x2 does not give x4, it gives x3 (base xp plus two times 100% of the base xp extra).
So,
base xp (x1) plus vitality (x1 base xp) plus nevit rune (x1 base xp) plus xp rune (x 0.5 base xp) gives 3.5 times normal xp (1+1+1+0.5). If you cannot use the nevit rune because you're too high lvl, 2.5 times is max (vit+rune).
Not sure how the mentee buff stacks, I do not have it. Should be easy to calculate though, since every mob states explicitly in the system chat how much total xp and how much bonus xp it gives. So just subtract the bonus xp from the total and then you know what the standard base xp would have been. Then divide the bonus xp by that, and you know your current multiplier for the xp.