Engine backfires and when put in gear dies

You don't say what make or model the vehicle is so this is somewhat general; Many auto trannies have a vacuum assisted modulator valve that helps "cushion" the shift from one gear to another and since you hear a vacuum sound, I would check to see if you have a break in the vacuum hose that runs to the modulator. If you look at the tranny (and it does indeed have a vacuum type modulator) look for a vacuum hose attached to the tranny and inspect it to make sure it is intact and connected. Both at the tranny and the intake manifold. It is also possible for some modulators to have a vacuum leak internally even to the point that I have seen some actually suck tranny fluid out and into the engine (hmmm. Possibly your low tranny fluid condition). If that is the case you may not beable to visually confirm it. But it would answer not only the vacuum sound but the power loss as well. Besides, it is easy to replace and not too costly a fix. Check it out and good luck.