They are the only number two in their crew, or are recognized by the other No.2s in the same crew to be the strongest among the other No.2s in the same crew.
This really does not apply to Zoro, since Zoro is not the only No.2 (as refuted by Oda himself), and Sanji would never say something such as "combat wise Zoro is superior!", so the reason for choosing Zoro must have been a different one.
Notice what Oda is implying here;
Oda could have just said that Bepo was chosen because he is the strongest among the No.2s. But instead Oda deliberately chose to make it clear that in the case of the Heart Pirates there's no competition of No.2s disagreeing about who is the strongest among them; They all agree that combat wise Bepo is superior, which is totally different from what happens in the Straw Hat crew, in which neither Sanji nor Zoro would recognize that the other is stronger, regardless of who of them is in fact the stronger one.
According to the structure of the text, since Oda was only referring to not holding the position of vice captain when he wrote "same goes for Zoro", and since Oda did not specify that Bepo's reason for being chosen was the same reason why Zoro was chosen, it's still open to our imagination why exactly Zoro was chosen among the No.2s.
I would say it's a lot more likely that it was due to his popularity, which is by far bigger than Sanji's.
