Viper is the easiest job to play if you know how to organize your (cross)hotbar efficiently, but a nightmare to those who let the game do it for them.

Let me help you:

1. Place Vipersight UI at bottom center.

2. Set to Hotbar the weaponskill Steel Fangs by the left side of Vipersight and Reaving Fangs at the right side.
Now it's positioned accordingly.

3. Now memorize this: if you start the combo with the right sword (Reaving Fangs), it will be a "Rear finisher", if you start with the left sword (Steel Fangs), it will be a "Flank Combo".
The following combo is always a inverted version what you just did.