When Does Vigilantes Take Place?

Not long after All Might's injury, so about 6 years before the current arc of the manga. So events from the main manga will probably never have a bearing on this, short of a timeskip.

How long before MHA is Vigilantes set?

My Hero Academia: Vigilantes takes place five years before the events of the main series, so there have been a few cameos of pros and other characters here and there but the Aizawa arc changed things.

What year is MHA Vigilantes set in?

So, using this piece of information, I feel (mostly) confident in saying the majority of Vigilantes takes place in Year 1 AB, and current arc in Year 4 AB (so 1 year after the fight and 1 year before Midoryia meets All Might). This would also mean the underground flashback would be in Year 3 BB (Before the Battle).