Well the title says it all.
In the first Battlefront, when all the command posts were captured the command vehicles (AT-AT's AT-TE's and shield generator) had the to be destroyed to set off the count down timer until victory.
So the problem is that in Battlefront II, command vehicles do not have an effect on the timer when all the command posts have been captured.
I have looked in both Battlefront I &II mood tools asset files for these vehicles and they are all have the same odf with no different in the command post values.

So I was wondering is there a way I can do this that I don't know about or is it hard coded into the game already?

Have you tried adding the vehicles to the objective? (If it doesn't work then try adding the vehicle without a vehicle spawn [so place it as an object] and add it to the objective, shield generator should work by simply adding to the objective)