I've been playing the engineer and when I shoot an RPG at a tank, sometimes I get the damage bonus notification, sometimes I don't.  I know in the past games and in other games, the tanks have more armor up front, so you want to shoot them in the back, but I've shot tanks in the back haven't gotten the notification.  The only time I seen to get it is if I manage to hit the turret.  Also, how many shots fired from the default RPG does it take to blow up a tank (and where is it hitting)?  For bonus, how many C4 changes does it take a blow up a tank (and does it matter where the charges are)?

The back, but yeah I dunno sometimes I get the hit notification and sometimes not.  I think it's like four RPGs to kill a tank, but that's just from hitting the side mostly I guess.

The ass.

There are flaps all around the tank which shield the actual tank HP (I think).
 
I try to set C4 on the backs of treads or on areas that are fucked up already. A lot of the time I just have to lob em blind so I don't get killed =)

The underbelly. Try to aim in back of the tank, below the chassis. The top is also very vulnerable.
 
Also, takes two c4 to explode a tank.

In most games I think the back of the tank is the most vulnerable.

I think you'll get the bonus notification when the tanks HP has been drained 50% or something like that. I seem to get the notification completly random. But what I've noticed is that the HP of the tank is considerably low when I get the bonus.

" The underbelly. Try to aim in back of the tank, below the chassis. The top is also very vulnerable.  Also, takes two c4 to explode a tank. "

In past BF games the weakness was always at the back near the vents.
 
But for this game, I don't know why but even shots to the back seem to do the same amount of damage.

As spaceturtle said; you only get this if a specific amount of damage has been done to the vehicle. A certain "weak spot"-hit won't give you this. It will definitely deal more damage from the back and thus increase your chance of getting the bonus.

the tracks (wheels)

in real life it's always the ass. Where the engine and/or ammunition is and also the weakest armor. Tanks are meant to be offensive so all their armor is in the front.

I have a tank question, if you jump on a tank, can you throw a grenade down the manhole and kill it?

" I have a tank question, if you jump on a tank, can you throw a grenade down the manhole and kill it? "

" I've been playing the engineer and when I shoot an RPG at a tank, sometimes I get the damage bonus notification, sometimes I don't.  I know in the past games and in other games, the tanks have more armor up front, so you want to shoot them in the back, but I've shot tanks in the back haven't gotten the notification.  The only time I seen to get it is if I manage to hit the turret.  Also, how many shots fired from the default RPG does it take to blow up a tank (and where is it hitting)?  For bonus, how many C4 changes does it take a blow up a tank (and does it matter where the charges are)? "

The underbelly

In addition, armour only takes full damage from a rocket if it takes a direct hit.  
 
Skimming hits or glancing blows will do reduced damage. Eg. if you aim for the side of a tank from a sharp angle and barely hit it, it'll do damage, but not that much. This was actually in BF1942 and brought back for BC2. They didn't make a big thing of it though. You need decent aim with the rockets now. 
 
But yes, the vents at the back are the weakest, whilst the back generally second weakest. Side takes the next largest amount of damage, whilst the front has heavy armour. 
 
C4 planted on the under-side should 1 hit kill, as far as I remember. 2 to the vent will also kill.