Viper skill page is a complete mess.

[Jag227]

(CONTINUED IN REPLY TO THIS POST)

Now that we understand the theory, we can simplify the rotation to simple 1 or 2 button inputs. This rotation properly rotates between the 4 finisher skills. Full rotation:
1 - 1 - 1
1 - 2 - 1
1 - 1 - 2
1 - 2 - 2
REPEAT

To prove this is correct, place the 2 combo buttons (Steel Fang on 1, Dread Fang on 2 on your bar and press them according to this order. You will see this is the correct sequence of the "hit the glowy buttons" theory.

Now, notice how we never once used the skill Dread Fang - button option 2 during Step 1. This skill applies a 10% Damage Taken debuff to an enemy for 20 seconds (a very important ST damage debuff!) However, adding this skill into our rotation complicates it to an unimaginable degree. This skill is the reason Viper appears so incomprehensible. Watch what happens when we account for this skill. Since the debuff lasts for 20 seconds, it must be applied every 3rd round of our rotation (for optimal setup and 100% uptime)."2" in Step 1 represents "Dread Fang" The full rotation becomes:

2 - 1 - 1
1 - 2 - 1
1 - 1 - 2
2 - 2 - 2
1 - 1 - 1
1 - 2 - 1
2 - 1 - 2
1 - 2 - 2
1 - 1 - 1
2 - 2 - 1
1 - 1 - 2
1 - 2 - 2
2 - 1 - 1 (here for illustration, not meant to be used before Round 1)
REPEAT

This is what happens when you account for Dread Fang and maintain the pattern of alternating Step 2 every round and Step 3 every other round. Notice how we don't arrive back at our first round (2 - 1 - 1) for so long. Dread Fang alone transforms a 4-round rotation into a 12-round rotation.

(EDIT: The following is incorrect and corrected by @Shurrikhan)
Now, the debuff provided by Dread Fang - Noxious Gash - can be provided by another skill available to Viper - Dreadwinder. This skill holds 2 charges and refills every 40 seconds. Since each charge applies a 20s debuff, and we have 2 charges, we can achieve 100% uptime on the Noxious Gash debuff simply by using Dreadwinder ASAP when it becomes available. Also, this skill starts a separate combo chain which is far more powerful than our basic weapon combo anyway, so this leads to maximum damage potential.

From this, I can only conclude that Dread Fang should never be used. It is a wasted skill and only exists as a necessary placeholder skill for the 3 potential future skills that it morphs into as part of this single-button combo system implemented in this Job design. With the conclusion that this skill should never be cast, the Job becomes a lot more comprehensible to me. (Of course, I am exaggerating when I say it should "never be used". It can have utility when used to reapply that debuff to a new target on short notice.)