You do realise Vettel was on vastly newer tires and had been gaining on him by 5-7/10ths a lap for what 10-15 laps and Ricciardo was gaining at 1.5seconds a lap or more on Verstappen.
Once the gap was closed, which it was when Verstappen went off, he didn't have anywhere near the tire performance to maintain any kind of gap to Vettel. It was like 55-60 lap old tires vs 35-40 lap old tires with Vettel and 20 lap old tires to Ricciardo.
I was looking at the sector times, I realise he was defending but he was before he went off too, and wasn't so slow then. Unless his tires went off the cliff at that point and he was genuinely much slower and it looked like he was putting the team first.