i know i can use any element but for pvp does any element give any specific bonus also would it be better to just use piercing since it apparently ignores defence
You will still have a lot of benefit choosing an element and upgrading it past 120. Elemental damage is not calculated in your CR, but gives a rather large damage boost as a separate elemental damage percentage, added to your attacks if you give your weapon that same elemental through charms, or if the weapon has any native elemental.
While I cannot tell you which would be the "best" element for Vamp, I gave fire to mine. Fire procs are basic and direct. Your usual general RPG fire DoT. No idea if these DoTs work in pvp, but just having your character engulfed in fire and losing HP can be distracting enough. So if elemental procs do work in pvp, that's a mind tactic to keep in your arsenal. Dark's proc is a move lock, represented in dark chains. Characters can attack but cannot move or even turn around. Poison procs a "weakened" debuff that seems to be bugged. At least I know others haven't found what it exactly does. Ice freezes once in awhile. May be good in pvp, but in pve, if you freeze a boss in the middle of his one-shot, be prepared to stall your i-frame until the freeze ends. Lightning does a sort of stun that lasts a few seconds, zapping smaller mobs in place.
As for the other damage types (pierce, slash and crush) I haven't seen anything outside soul gems to buff them, so the amount of points you can give to them may be minuscule anyway. Also, if you check your character's skill set, it will have skills with different types. For a Vamp specific example, Bloody Hell, a VERY strong attack, is Piercing type, but Predation, which is her level 65 Ultimate, is Slashing. So choosing one type would only benefit a few skills and not all of them. While buffing a single element and adding that element to the weapon will add elemental damage to all your attacks.
Other more pro players can give you better info, or most likely more correct info. But at least from my knowledge, it's better to invest in elements than in physical damage types.