Yeah, the magrider's ability is to put on a very large burst of speed, up to around 100 kph when I used it with no certs into it, which also lifts them off the ground a bit more so they can clear ridges easier. Also tank armor is thickest in front, remember. If you are shot in the rear you blow up way way faster.
The Vanguard's ability is to shield itself. It's great for driving to put cover between yourself and the enemy, or for when you need to turn when a magrider zooms past you. Remember Magriders don't have a turret; the whole tank is basically the turret as it's floating. That means if it's shooting you they have their best armor forward.
Magriders can also side-strafe with their best armor forward. When you drive one it takes a second to get used to it but it's a fairly nice design. The keys a and d strafe you sideways instead of operate a left or right track. It's a bit clunky to turn as it's limited to mouse movement but with a bit of practice it seems alright.
Also all the main battle tanks can unlock AP (Armor piercing) rounds which might explain your tank's combustibility. The downside is that it loses the High Explosive aspect of the default HEAT round' which means you are a bit less versatile. Once we are a few months in past release though and you've unlocked the anti-infantry secondary gun it probably won't be as much an issue though. That said I don't know if that anti-infantry secondary gun is any good as I haven't tried it yet so don't take my word for it as of now.
Also for blowing up infantry you'd want the HE cannon. That would most likely give you the one shot you desire if you land it square, though all the infantry classes seemingly can unlock FLAK armor which would less the accuracy of that statement a bit. HEAT is more for versatility which is good if you have some support.