I suggest reading this thread:
http://www.dakkadakka.com/dakkaforum/posts/list/30/223186.page
While the basic rules for attacking a vehicle seem to give two possible options when attacking a vehicle that is currently immobilized but which also moved in its previous turn, when you also factor in what the rules for 'assaulting a fast vehicle' on page 70 of the rulebook say you realize that it is pretty darn clear:
As soon as a vehicle is immobilized then attacks in
CC hit automatically.
So yes, if you immobilize a vehicle at I10, attacks at a lower Initiative value would then hit automatically.