Maybe because Saving Private Ryan was a better film, which had a good cast and array of characters to empathise and asociate with.

Rambo, while fun. is really nothing to write home about unless you are into the "hurr durr i did da mostest damage with ma gun!" kind of game.

 Someone's sarcasm detector needs a recalibration… 

Cifer said:

Well the use of sarcasm (which largely depends on inflection and mannerisms in its successful delivery) is so useful in text format on the intertubes… oh whoops.

Back to the OP instead of talking about some movies.Taking out an enemy vehicle could indeed make a very nice mission or part of one. Though going head to head against one is a folly. Most vehicle mountd weaponry will make mincemeat of any pc and you only have so much fatepoints to cheat death. To take one out, would require the use of intelligence. Setting an ambush, a trap or a sabotage mission could win you the day instead. 

Recomended playing:

Battlefield 3, in a Tank, on a city map, without using 3rd person camera.
Unless you know exactly what you are doing you'll quickly find out why tanks need to be supported by infrantry in close quaters battles. Furthermore even in hardcore mode BF3 is still an arcade-ish game, especially when it comes to vehicles. The tanks accelarates, turns and reverses far faster than a real tank would. But still it's an interesting experince when your field of view is limited (which is even more true for WW1 and 2 style tanks that the imperiums tanks a based on) and you can be attacked from almost every angle.

I'd probably handle an encounter with a vehicle (and some soldier that are with it) kinda like this:

The players somehow become aware of the vehicles existance (i.e. they hear it crush some fortficiations nearby or encounter the broken remains of a squad of soldiders that were attacked by the metal beast).

In narrative time the players probably formulate some sort of attack strategy that ends with them either being detected before they can significantely damage the vehicle or with them using explosives, rockets or whatever to get the "first strike" against the vehicle. Before this happens they may run into some sort of encounter with the soldiers scouting around the vehicle or whatever.

So either: direct encounter with the vehicle because the PCs failed to do "something" or they get the first hit against the vehicle.

In either case next would be some sort of combat encounter, depending on the PCs succesfullnes the vehicle could be fully operational, unable to move, unable to shoot its main gun thereby turning into "just" a fortified machine gune, or the vehicle completly disabled.

Assuming the vehicle is fully operational the encounter would probably end in a lot of fate points burned or generally unsuccesful.

I think that a GM should always allow for the players to act before the vehicle becomes a direct threat. I.E. a vehicle should never "sneak up" on a party. A tank moving whether moving through a city or an open landscape would always be spotted before it notices any infrantry. Whether by sound, line of sight or combat reports from other units. I think a vehicle should force the players into a situation where they have to use their surroundings, their skills and smart play to overcome something that would easily crush them in direct combat. After all, direct combat is what most combat vehicles are made for.

As the PCs are infantry they are more mobile and flexible than a single tank. Everyone spreads out, uses cover to get around to sides / back of vehicle and hit it were it is vulnerable. Any one who attracts the attention of the tanks will probably die, but should buy enough time for the rest of the squad to take out the tank - that would be the reality of Guardsmen taking on a tank. If you can get the drop on vehicle, then you should be able to hit it first, so areas with lots of cover like cities would be perfect for taking care of tanks without infantry support. Undoubtably at least someone in the squad will have some weapon capable of anti-tank duties, particularly if you get to call shots at weak points. Bearing in mind we are dealing with 'normal' humans as PCs, it would be very unrealistic for them to deal with numerous battle tanks all at once without serious assists from NPCs.

Scribbs said:

Perhaps this should read: If the Pc's are infantry….

Ok '…if the PCs are infantry..', fair enough comment. Playing as part of a tank crew is obviously very feasible for OW, but I'll confess that in my mind the core experience of being in the Imperial Guard is to be a poor grunt in a very hostile universe. You're pretty much at the bottom of the pecking order, with every xenos stronger, faster,more teche- up or just out-right nastier than you. The goal is not so much beating the enemies of the Imperium so much as just surviving. I'm sure plenty of people will disagree with this, but that's just my opinon. I think the best war films are more about the comradeship at arms and getting out alive from conflicts rather than battlefield heroics (although both things are obviously not mutually exclusive). I think OW will give a real chance of capturing that dynamic in the 40K setting.

 While 40k IG infantry is arguably one of the weakest individually they are VERY dangerous in numbers and cheaper (Point wise) than any other unit. Also, While IG infantry may have their weaknesses IG armor is unmatched by anything out there except maybe the Necron Monolith! 

Ever since the American Civil War, when long-ranged, accurate guns became common issue, infantry's mantra has become "stealth is wealth." Or, more literally, "stealth is life." Doubly so when mobile fortresses (armored fighting vehicles) are about.

To summarize a lot of the wisdom already mentioned on this thread, the smartest doctrine when facing vehicles is: 1) Hide; maybe it'll go away. 2) Call up someone who's equipped to deal with the nuisance; i,e., tanks, tank destroyers, artillery, air strikes, et al. 3) If all else fails, fight as dirty as possible. Get close; AFVs are open-country monsters. Use mines if you have them (or IEDs; look at Iraq). Do not rush, yelling (after 2004, that tactic utterly stopped in Iraq, since those who practiced that tactic failed to survive it).