the vanguard mode of Dark avenger is helpfull in pvp and pve???Is Dark avenger in vanguard mode damage dealer??and what about panther in gracia??

vanguard is pure fail. You gain minimal p atack and 2 skills which are not usefull or powerfull at all at cost of shitloads of p def and losing shield block. Also you cant use your regular skills in that transform (except some few, depends on tank type you are)

about panther, i heard rumors it will gain some skills in gracia final or so (i think it is surrender unholy, invisibility, shadow strike and kind of backstab or so)

--> but im really unsure bout it, anyway in gracia 2 it should be same panther as in older chronicles

Well, yea. Vanguard kinda failed.
I mean, ok, you can use Dual, or two handed sword, but in pvp it sucks, in pve, you can lvl up with panther, so it sucks.

WITH SK/TK it may be useful.
With SK i liked it, because the new skill animations were nice to me xD