Vanguards are usually seen as that wild, reckless class full of ADHD-riddled players who love their suicide charges; I admit, even as one myself, I always get a bit uneasy when I see fellow Vanguards in the game lobby. It took me a long time to learn how to play Vanguard properly in ME2...and I practiced quite a bit in the ME3 single-player demo before I felt I was fit to assist my teammates properly in MP.
Fellas, here's what a good Vanguard will do, and this is how you can support them:
A good Vanguard will look before they charge. If there's a Vanguard ducking in cover and not doing any shooting for a second or two? Chances are they're scoping the area for charging opportunity, and if none is present, often they'll move somewhere else and try to flank the enemy. Vanguards rarely need actual cover for reasons other than to restore their barriers/cooltimes and seek charging opportunities; personally, I always give up cover to my teammates if I'm just doing the latter.
Good Vanguards do charge, and they do it a lot. You might be asking, why the hell did that Vanguard charge into that enemy that was six feet in front of him/her? The quick answer: it keeps us alive. We get partial (or full) barrier restoration upon a successful charge; it's our main method of staying alive. We also like to charge for quick access to a higher or lower level, skipping the irritating (and possibly dangerous) ladders and drops. Other times, charging that Guardian and then quickly circling around him his the perfect way to get him to turn to us, exposing his soft, pliable back to the rest of the squad. Or some palooka is about to execute a downed teammate - until we fling him into deep space with a charge. I know it's probably annoying when a Vanguard suddenly charges into an enemy you were targeting, knocking them out of your crosshairs, and I apologize for that; but charging is an absolute necessity for our survival.
Good Vanguards won't run off to go Rambo. We're decent enough on our own, but far better in a team; we can knock down enemies with Shockwave, Charge, or Nova, and teammates can mow them down before we've even finished the animation. An Engineer's drone plus a Vanguard is a fantastic way to manage aggro. Basically, if you know how to play your class, a good Vanguard will know how to play theirs so that everyone can properly wreck collective ass.
One more thing: A good Vanguard will not want you to risk your neck to revive them. Sometimes we die no matter how careful we were with our charges, and more often than not, it's in the thick of enemy fire. Please leave us to rot - we're no strangers to dying like that, believe me, and most of us can live with the fact that we made a mistake and are now paying for it. Team surivival always > our survival.
(Disclaimer: AlphaDormante is only one Vanguard of many, and despite her usage of plural pronouns, does not intend to speak for all Vanguards and is more or less saying how she plays it :innocent:)