
I have found that Assaulting Vehicles is generally the best way to deal with them. Through much game experience, I have noticed that vehicles are extremely difficult to destroy through shooting. Even the weapons that are praised as "Anti-Tank" have difficulty finishing the job.
However, assaulting vehicles has proved to be more fruitful. The reasons being as follows:
*You will generally get more dice to roll. Even a tactical squad using Krak grenades gets 10 attacks, whereas the meltagun, lascannon, or missile launcher in the squad only has one shot. Since I 'grew up' in 40k playing Imperial Guard, I always believe that more dice is better. I have not even mentioned the fact that you get your full attacks when not using grenades. We all know that more dice rolled to hit means more dice rolled for armor penetration. More dice for armor penetration means more dice for the damage table.
*Your attacks are directed at the rear armor. I find it easier to reach the front of a vehicle for an assault against the rear armor, rather than manuevering behind a vehicle for shooting attacks. My opponents are generally very careful about where they point there vulnerable rear armor.
So, I encourage you to assault those vehicles. Especially if you can catch them driving at combat speed or stationary. If the vehicle didn't move, you can't miss.
Since marines come standard with Krak grenades, do it. According to the new Codex, the regular guardsmen comes standard with Frag grenades. So, the lowly guardsmen is not too weak for the AV 10.
After considering this further, the guard army majors on firepower. So, you guard players may do well without having to resort to assaulting vehicles.