Fight as Shin Akuma/Super Gouki

Highlight Akuma/Gouki at the character selection screen. Then hold Start and press A

Fight as Super Vega
Select arcade, training, or versus mode. Highlight Vega at the character selection screen, then hold Start and press A

Select winning pose
When you get a KO, immediately hold a Punch or Kick button. Different buttons will give you different poses. Note: Different characters have different number of poses.