VTA Cartridge

Your cartridge is attached to a flat plate.  That is what should be parallel to the disk as a starting point. 

Usually, but not always, that plate is parallel to a a linear extension of a line bisecting the upper and lower lines of your conical arm.  You can often see that line by shining a bright flashlight on the arm from the side.

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.

Front face= VTA. Sides=azimuth. 

Visually level, front face perpendicular, is an okay starting point. VTA however can only really be set by ear.

Soundsmith has a great video, Lederman explains VTA better than anyone. Yes even better than Fremer. The trick is to just play some records, listen close, tweak VTA up, listen again, tweak VTA down, listen again. Through the combination of watching the video to understand what's going on, and listening, you will be able to fairly quickly figure it out, know which way to go, and hear for yourself when it is really dialed in.

When listening for VTA, notice with every note whether plucking a guitar string or whatever, there is the attack or beginning, then the full tone or body, finally the decay. When VTA is off one or the other of these will stand out. This sounds goofy at first but when you do it you will see. Raising the arm changes the sound one way, lowering another, and once you know which way to go you'll be able to raise (or lower) a tiny amount each time and as long as it keeps getting better you keep going. Eventually you will hear it change for the worse, then you go back the other way a bit. At that point either you are done, or you are hooked and become one of those guys who loves to do this with every record.

Or you just set it level and forget it. Your system, your time, your ears... your call.

Audiosens, If the front face is perpendicular to the mounting surface then yes, you can use it as a reference. VTA is not all that critical. All you have to do is get in the ballpark (contrary to mythological opinion.) Azimuth is way more important. The stylus has to be perfectly perpendicular to the groove as seen head on.  

The stylus angle, azimuth and SRA, determines tracking, not azimuth.

Azimuth is the relationship of the generator to the record grooves. The stylus can be perfectly aligned but if the cantilever is twisted, or the coils, or magnets, then azimuth will be off. Which you have no way of knowing because these are all hidden deep inside the cartridge.

You know this is all true. Its just that nobody has ever bothered to explain it before. Or you just never bothered to think about it.

So think about it. Please.

Because you need to realize, you cannot go by the stylus. You cannot go by the cantilever. You cannot even go by the cartridge body. Azimuth can only be set by ear, or by instrument, but not by eye. Absolutely cannot be done.

But hey, don’t take my word for it. Go to Soundsmith website or YouTube channel, watch Lederman explain in detail exactly what I just said.

Please.

Oh and btw, factually heard is neither mythological nor opinion.