At first I thought that Ian arranged a false alibi for Vincent because they were both part of a pedophile ring and if Vincent was interviewed by the police he might spill the beans.
Now I wonder whether it is also, or alternatively because the man with white/grey hair who watched Tony and Ollie playing at the pool is the same person as the man with white/grey hair driving the cleaning van later videoed outside the house where Ollie was kept.
That is the man with white/grey hair was involved in Ollie's disappearance. He might have been seen by Vincent who as we know was in fact loitering by the pool and lake area at the time of Ollie's disappearance. I don't think at the moment that Vincent was involved in Ollie's disappearance, but if he was in the area then he might have seen the man and been able to identify him.
So Ian isn't protecting Vincent from being a suspect in the disappearance but trying to ensure that the police don't interview a crucial witness.
James Nesbitt's acting was superb as Ian told his guilt is a cancer anecdote. He looked like he was about to be sick as he realised but at the same time he looked like someone trying not to react so that Ian wouldn't realise that he'd twigged he knew Vincent.
The bit about the worst day of Tony and Emily's life being the best day of the journalist's life (because he got his big break) and the best day of Mark's life (because he met Emily) was quite chilling. Does Mark really see it as the best day of his life? Surely you can't really love someone and think that?
Thanks to the poster with the theory that grieving parents Ian and Ian's wife have taken Ollie. That is about the best case senario I think.
There are quite a few lost children now:
Ollie
Ian's daughter - if he's being truthful about her
Baptiste's daughter - who is lost in the sense that they have lost the daughter they knew to drug addiction.
The other thing about Ian is he doesn't actually say how old his daughter was when she disappeared. We are led to think that she is very young because the portrait is, but I wondered whether she was a teenager or older, as the way Ian talks about the police assuming that she'd run away is not the way they'd deal with the disappearance of a very young child. And if she'd disappeared as a young child then wouldn't Tony remember the case or recognise the photo?