Vigilante: Boku no Hero Academia Illegals - Can I jump straight in without watching a single episode of the main series?

i think the answer is yes, because its a difrent story in the same universe as the main series, but its kinda like a prequel spin-off in terms of time-line, some heros from the main series appears in the spin-off but its ok if you dont have watched the main series, you just dont know much about them but for the spin-off that doesn't really matter.

I hope i have helped you

Pretty much yes - there'll be quite a lot of cameos and main series people appearing which you might not get, but not in any big context that actually relates to the main story so it won't matter much.

yeah, pretty much.

I would recommend watching a season or two of the main series just so you have an understanding of the world and some of the characters in it. Like others said it's a prequel with new characters so its unlikely that it will spoil anything from the main series or hard require knowledge from it, but it's nice to recognize pro heroes when they show up and have some other context.

In any case if you liked the first episode of Vigilantes you'll like the main series for sure!

Yes. Illegals is a prequel of the main story as Deku narrating in the beginning says and makes question to highlight

But you'll lose some of the easter eggs and references to the main series without knowing but it's "spoiler-free"