To find Vengarl you need to have already freed Rosebeth from petrification.
If you continue progressing after she is moved you will come to the bonfire "Ruined Fork Road" in the Shaded Woods area where there are three exits in front of you. Take the left exit into the Shaded Woods area. Stick to the left of the area area and you will come to a left turn, follow this to find the head of Vengral. (Be careful as there are invisible enemies in this area that might follow you).
Once you find Vengral you need to exhaust all his dialogue until he gives you his helm and this is when the trophy will pop and he will be unlocked as a merchant.
He also gives you a gesture needed for another trophy.