Solid question.
Not specifically speaking.
https://beerandbrewing.com/dictionary/zWFF6aqpxT/#:~:text=Vorlauf is German for “recirculation,under the vessel's false bottom.
Whether you’re doing a single infusion, decoction or step mash, these steps with recirculation are mostly to keep the grain bed a more consistent temperature as well as increase the efficiency/extraction during the mash. When you’re no longer extracting sugars - I “confirm” this with 2 consecutive same readings on the refractometer, you move on to the mash out at a higher temp - generally 168-170 and vourlauf. This may be a slightly different “home brewer’s” understanding but like government work; it’s not just good, it’s good enough.
Were you to mash say, black patent malt for the 75-90 minutes, mash out and vourlauf along with the other grains, it’s definitely going to contribute a bitter astringent quality to the beer. That will definitely detract from drinkability.
So, Brewer’s Friend recipe calculator allows you to specify a late addition on your malts, which will help keep you “in the green” numbers wise.
There are a couple ways I know of to reduce / avoid astringency from very dark malts.
Late addition like you’re discussing. I usually go late in the mash though (because)
Use special dark malts - Blackprinz for example instead of black patent. Carafa specials (2 & 3 I think) and so on.
If all I had were Black / black patent malt, I’d add as late as I could and recirculate only for color and go!
Although not specifically to style, I like my Porters BLACK, like my soul... So I keep a handy supply of blackprinz.
There is a recent all in one help thread < cannot search for it using “all in one” it gets rejected for too few search arguments >
It has a lot of good info on process and procedures for the various all in one systems. I bring this up because you mention the 60 minute mash. I’ve got both an anvil 10 and 18 and I’ve never finished a mash in 60 minutes ;-) Check that thread out as time permits.
As for when specifically to add darker malts to avoid astringency I’d say the last 10-15 minutes-ish of your entire mash/mash out/vourlauf process.