Well we may have to agree to disagree on that one - I definitely saw Son several times in his own half right by the left hand touchline either chasing an opponent, or trying to tackle one (normally failing, but that's a different matter). Of course he offers more as a forward than Vinny, that goes without saying surely, but he was also in positions right near the touchline deep in our half in places that Vinny would never have got within 30 feet of. So (theoretically at least) benefits in both attacking and defending (or at least slowing them down on that flank, where they caused Reg so many problems).It’s really not that complicated. Vinicius was having a horrible match and played just 3 passes, he was killing our attacks whenever he got involved by either the ball bouncing off him or being offside.
We put Son on because he is a much better forward than Vinicius, not to help Reguilon defend. I didn’t see Son do any defending.
Anyway, whether the potential defensive assistance was a factor or not, going back through to the start of this specific conversation, I don't think it was a surprising, bizarre or 'poor' decision by Mourinho to sub Vinny off and Son on, as was being suggested. However you look at it, whatever the drivers, it made good sense at the time, that's the main point.