The +13 enchantment bonus wont stack with the same dmg type as the base weapon type i.e. the piercing so it can be better to unforge that and add a regular 2d8 of differing type.
However as you have 5 different dmg types and if you start unforging you can only forge 4 dmg types, my advice would be to leave it alone - there might be others who can do the maths and advise better than i.... *coughs*KC*coughs*
If it's for a melee toon i would personally apply all legendary metals except the bludgeoning one (admantine?) as that would over write it with the inferior 4d6(?).
Unless you've eyes on a particular set weapon, I'd suggest it is good enough to forge up (I assume that's a super-lucky roll on a dm event item?) and remind everyone that dm's can do legendary resets for a small price (1mil gp and 10 wish shards iirc).
I'd wait on the vamp regen for the better runes as you can forge 10 vamp regen with craft weapon at 20 at the UD forge.
Runes/legendary metals always over-write anything already there of the same type.
You will probably want Teleport (TDD) on your final armor - i'd suggest magic find be placed on an armor many toons could use to lvl with.
If i haven't answered a question, it's because i'm not sure of the correct answer