VACNet rarely sends legit cheaters to Overwatch. It is simply an AI that scans for non-human movement / actions that spinbots replicate.
Most of the time, just legit cheating will not get you into Overwatch with the courtesy of VACNet, as Valve intends that only for blatant cheaters.
Regarding Overwatch in general,
Yes there is a myth floating around that going below 100 Score or 40 Kills will not get you into Overwatch and vice-versa, I can confirm with my own experience that going over 40 Kills or 100 Score had no effect on me whatsoever, as I go above those “limits” very often.
As some have already mentioned, no one really knows. If Valve released a criteria on when you get into Overwatch, it would be abused in a very bad way. Any answer is just speculation unless datamining discovers something that could prove it.