Guild Wars 2

Blood Magic Master trait
Vampiric – Siphon health whenever you hit a foe.

Blood Magic Adept trait
Siphon health whenever you critical hit

I just want to confirm.. if you hit a critical, you will trigger BOTH of these? Or will one override the other?

Both will trigger, one for 25, the other for around 90. (40 and 135 with Bloodthirst)

It will trigger both.

If you also take vampiric rituals (wells siphon health) you can get 3 leeching proccs out of one well tick per enemy hit (if it crits).

The proccs are 25 dmg for Vampiric and 30 for Vampiric precision without bloodthirst.
The 90 damage stated by Panhauramix are probably from the 1s bleed procc from the curse traitline.