I had it drop recently from a expert lost sector. Cheers and luck getting it.

People have reported getting it in different sections of the Vanguard Ops - one said they got it from Arena, another said their friend got it from Pinnacle, and another said they got theirs from a Solo Op. One thing to keep in mind is that Acrius was one of the original set of Catalysts which were given a lower drop rate than catalysts that were introduced later as they were seen as something to chase over time. Bungie have mentioned that they had increased its drop rate above what it was back then when it was moved from Leviathan to Nightfalls, so if its still the same - you're looking at a potential hefty grind for it if you're unlucky.