Alonso actually did three stints (before the Red Flag).
He boxed twice (before the race was suspended).
Alonso stops were:
Lap 17 (he changed to Softs - the slower tyre, to get them out of the way)
Lap 34 (he changed to Super-Softs - the faster tyre; known to go at least 40 laps, even more on lessening fuel loads)
So, he was closing in on Vettel during his third stint...on the faster Super-Softs.
They then all changed on the Red Flag Grid.
Cheers.