|Getting a good seam finish is so important that I think it's worth buying a sample piece of fabric to test before even buying the project fabric. Failing that, test on scraps to be sure of a safe method.
I found this works when the velvet will take steam:
The idea is to get the steam doing the work rather than pressure. The pile side should lay on a thick towel. I use a seam stick (length of wood, half-circle in profile), which is laid on the ironing board, covered with the towel. The seam is positioned to run along this curved surface, and finger-pressed open, then brown paper strips placed under each seam allowance. I lay a damp (not wet) press cloth over and use the iron barely touching the press cloth, to generate the steam.