Can models which have been Knocked Down use the 3" move from Vengeance to remove Knocked Down?
Can models which have been Knocked Down use the 3" move from Vengeance to remove Knocked Down?
No, you have to forfeit your normal movement / action to stand up. Vengeance are neither of those.
No. While knocked down, a model cannot advance. A model cannot sacrifice movement/action to stand until the start of its activation, which is well after the Maintenance and Control phases.