They were just into T2, the field had not sorted itself at all, and there were still many side by side pairs; the lead car made its approach to the apex from the full left side as one would if they were not under any pressure. We can’t know fault without more video, but so very early in the race its critical to watch the mirror and know what’s around you when you make the turn in to apex especially when it starts from full stage left. It’s easy and maybe even likely to say the fault is with the other car, but with just this view who can say?
Leave racing room is one mandate, as is, the lead car owns the corner, as is, make a safe pass, as is, watch your mirrors since there may be someone significantly faster. With only this video we can’t say who made the error since we can’t really know where this other guy was for the run up to the corner. I trust everyone is ok, dented cars and dented bank accounts are bad enough.
As someone once joked: Everyone slower than me is stupid; everyone faster is crazy. Which are you?
After watching it again several scenarios could have taken place. Possibly the yellow car may have way overslowed but judging from the gap to the cars in front of him it doesn't look like it unless they were all being conservative on the first few turns. The car that hit him may be a lot faster or much better driver who can take turn 3 much faster but his line was not the best if so, he could have come in too hot from being too amped up and not thinking which is my guess. But in any event if he is that much faster the wiser move especially in vintage would be to pass him on the straightaway, it was only the 2nd turn of the race.