To begin with, based on his description (see above), I think the OP is conflating azimuth with VTA:
"Could I consider that the front face of my cartidge (DV20x2H) is perpendicular (90 degree) relative to the vinyle surface, can we concider that it is a way to have a good VTA ?"
This seems to me to be a reference to azimuth, not VTA at all. Also, as regards VTA, the fact that a tonearm wand is cylindrical is in no way a hindrance for setting VTA. (To the OP:) To set VTA, you view the arm tube from the side and with respect to the surface of an LP sitting on the platter. Viewed from the side, a cylinder will present two straight parallel lines, just as if it were square. If you make those two parallel lines parallel to the LP surface, for most styli, that is a good starting point for setting VTA. But, like Raul says, you tune by ear from that starting point. Better yet, start by setting the top of the headshell parallel to the LP surface. That's for VTA.
"Could I consider that the front face of my cartidge (DV20x2H) is perpendicular (90 degree) relative to the vinyle surface, can we concider that it is a way to have a good VTA ?"
This seems to me to be a reference to azimuth, not VTA at all. Also, as regards VTA, the fact that a tonearm wand is cylindrical is in no way a hindrance for setting VTA. (To the OP:) To set VTA, you view the arm tube from the side and with respect to the surface of an LP sitting on the platter. Viewed from the side, a cylinder will present two straight parallel lines, just as if it were square. If you make those two parallel lines parallel to the LP surface, for most styli, that is a good starting point for setting VTA. But, like Raul says, you tune by ear from that starting point. Better yet, start by setting the top of the headshell parallel to the LP surface. That's for VTA.