This is not a viable method of reliably acquiring legendaries and never will be. Don't do anything that requires you to rely on RNG for progression if it's frustrating you. Even if you get a legendary engram from a strike, the odds of it being a directly usable item are less than 50%. save up and buy the gear you actually want.
At least you're earning vanguard marks so you could buy yourself one.
I have, but I've already used my Vanguard marks due to the weekly cap.