I have to admit it's tempting to let an idiot vanguard die, but I like to play as an infiltrator, which means I spend most of my time doing my own thing from a distance, and reviving people, so it's hard to tell whether it's an idiot vanguard, or a good one that got unlucky. But as for the ones who charge into a completely fresh atlas? No fucking way am I risking my arse for that. That's just stupid.
On another note, if you want to stick it to a vanguard, play a quarian infiltrator against geth on silver/gold. The vanguard gets nothing but the basic troops, because everything else is shielded, and if you sabotage them, either they'll kill each other for you, or explode when someone else does. Either way, you rack up the kills so fast, there is almost no way someone else will outscore you.