Wait what? I just played Field of Battle but the Vasudan Ace didn't jump in. 
That happens if you kill too many fighters in the previous missions. If your kill count from the previous mission is more than 5, the ace does not seem to appear.
Whaaaat?! 
Indeed. I ran into this same behavior way back in 3.6.9 and apparently it's still present. Tested it a few times.
- Killed 7 enemies in the previous mission, ace did not appear.
- Killed 6 enemies in the previous mission, ace did not appear.
- Killed 5 enemies in the previous mission, ace appeared.
They may be something in the code about that, but you would need to ask the people involved in that side about it. I'm just reporting what I've observed.
There's a very easy way to avoid this: Crank the difficulty up to Insane (or just merely Hard) and try getting more than 5 kills in the first mission. You'll be grinding that mission for a loooooooooooooong time.