I don't know of a specific way to farm them, but a way that I like doing it, and a way that can get your vanguard rank up, is by running the Dragon strike playlist which is the level 28 strike playlist. You're guaranteed a blue engram every time you do it, and you can also collect blues, or even a purple throughout doing the strike. Turn those in, and you could possibly get a purple out of a blue, and you can level up your cryptarch rank as well and get packages that could contain a purple. Hope this helps!