With Luffy as the captain I see multiple 2nd position spots on the crew due to the fact that no one of them perfectly fits the VC bill, if I base what a VC is off of Rayleigh.
Rayleigh is both extremely strong, instinctive, and intelligent making him a clear choice for VC. If i had to go by this for choosing VC for the SH's I would split up the job between Zoro and Nami/Usopp. Zoro would be a "Commander-in-chief" in terms of directing/advising the crew in it's military (combat) endeavors, however, he is nowhere near as clever or smart as Nami/Usopp. At this point I would cut Usopp out because Nami has a natural "bossy" edge to her making her comparable to Zoro in terms of fitting the job (minus all combat endeavors). In this way I believe Oda does not state VC because none of them perfectly match what a VC should be, and instead the job is filled by an ensemble of people (the crew) who contribute in whatever way the job sees fit.
Another aforementioned category brought up has been seniority. If this were the case I would still allow for any of the East Blue crew to be a viable option for VC, as they all joined extremely early, and the VC does not have to be the very first person who joined (Zoro).
However, if there had to be a definable VC for the crew I would say that Zoro is the VC as he has seniority (from the East Blue), leadership qualities (along with Nami), and he (unlike Nami) doesn't have an official duty for the crew besides "fight people".
I don't get it. Luffy is dumb as hell, but is the captain. I don't see the need of a clever VC. Care to explain?
Now about this discussion
Let's look at Luffy's definition of a captain:
Knowing this… If you have to pick one of the strawhats (other than Luffy) to fit this position, who would be?
Who keeps his head cool at crucial moments involving the crew?
Who gives Luffy the most important advices?
Anyone that didn't answer "Zoro" three times should read the manga again.