1° I have doubts about vocation (PVM) what is the best vocation for single target?

2° Does a comparison between a weapon with 20.0 atk and 21.0 atk make much difference?

3° Is character damage based only on skill or level influence? Is there any calculation for this?

1. Question is too wide. Do you mean vocation or sub vocation? At low-mid lvl depending on skill ranger could be the best vocation on single target pvm. At very high lvl warlock takes over.
note that right now its more about AOE in PVM. So if I were you I wouldn't concentrate on single target pvm vocations.

2. I believe its fair to say that 5% increase on wep atk makes 5-10% difference. Is it much or not its personal opinions. I believe its quite a big difference but not worth huge investments in the beginning of the game.

3. Once again depends if its spells or melee/distance weps.
Most spells depends on m lvl and lvls. Melee and distance dmg heavily depends on skill and just a little on lvls.

1.0 Atk = +25 average hit
I expected it to be in % of medium hit, the more level / skill the greater the damage increase for each weapon attack point

Zaanato (22.2 atk) to MSM (24.2 atk)
Level 171 - Skill 126
Average hit increased by 47.
Average hit 424 to 471

MSM (24.2 atk) for Ax (26.1) - Test server Axe creation
Level 270 - Skill 140
Increased average hit by 52.
Average hit 527 to 579

MSM (24.2 atk) for Ax (26.1) - Normal Server
Level 372 - Skill 152
Increased average hit by 49.
Average hit 671 to 720