Well, as long as the State of California doesn't pass the bill to ban .50 BMG rifles, one can get them, and ammo, on the civilian market, but AP ammo is a no-no. The jacketed .50 cal slug can go through quite a bit, though.

20mm grenade launcher? hm. maybe that's the caliber for the new OICW mini grenade launcher thing they want to strap on top of an M-16 frame. Normally GL's are 40mm (US) or 30mm (Russian/Chinese pattern.)

The Russian BMP is sensitive to flame attacks--its armor is made of magnesium! I think either it or the BMD (airmobile variant of the BMP) has its fuel tanks in the doors. Bad idea.

Armor varies widely. A .50 cal is typically enough to go through the armor of an older scout/recon variant, but might have a tougher time up against something like a Bradley. This is why a lot of armored cars have sloped armor, to increase the probability of round deflection--hardball AP rounds tend to bounce off of angled armor like that on a V-150, or even the similar BTR series. Sometimes armor is sloped for a particular effect--the South African "Buffel" APC has sloped underside armor and rides high off the ground, to reduce the effect of land mines and ground-based boobytraps, which were a common hazard when it was introduced.

Making a hole big enough can be challenging, and the other important factor is figuring out a way to convince the armored car to drive into said hole.

Armored cars with poor off-road capability would be more vulnerable to low-tech roadside obstacles like holes in the road, but vehicles with better off-road capabilities could either avoid the hole or would be more likely to be able to un-ditch with minimal problems.