Well, I tried a quick test, but the results weren't that encouraging.
I set up two groups of 3 tigers and 3 PSW 221s in scattered trees with about 500m between the groups and each individual vehicle being about 250m from its neighbor (in other words, two lines of six vehicles each). Next to the scattered trees the vehicles were in I put a swath of forest to break LOS, with the vehicle as close to the forest as possible while still being in the scattered trees. I then put Rifle 44 squads in the forest, with one squad per vehicle placed 100m away. The HQ units I stuck in a map corner out of the way. Conditions were clear, mid day, breeze.
One vehicle group was hidden while the other was not. I then had the infantry move 50m towards the vehicles. Almost as soon as the infranty squads stopped moving I received sound contacts on all vehicles. Repeating the test a few times, there seemed to be a tendancy for the hidden vehicles to take a few seconds longer to give a sound contact, sometimes up to 5 or 6 seconds. But it wasn't a rule -- occasionally one of the hidden vehicles would be the first to register. There also did not seem to be any distinction in how fast the two different types of vehicle appeared.
Repeating the test with strong winds acheived an identical result (I had thought the winds would affect sound, but apparently not). I also varied the distance the infranty moved. 35m gave no sound contacts, 45m gave contacts for all vehicles. I should probably have tried 40m, but I was getting tired of all that clicking.
I also tried seeing if hiding affected actual spotting distance, using move to contact, although this setup wasn't the best for testing that for a variety of reasons (particularly because the vehicles were all in profile to the infranty, hehe). Again, there were no differences. Ranges tended to be between 14m and 24m for both types of vehicles, hidden or not.
So, I'm not sure hiding a vehicle makes much of a difference at all. I ran this test perhaps a dozen times total, and only once had a vehicle not give a sound contact: one of the PSW 221s when the infranty moved 50m in strong winds. So perhaps hide vehicles just in case, but don't count on it.