Regarding the spread, I think it may not be that relevant. The vix ETNs are tracking the corresponding S&P VIX future indexes which are directly computed from the VIX futures. The spread of 1 cent for short term VIX futures ETNs may be much smaller than the spread for the underlying VIX futures but I guess you may have a "stale" price in those ETNs and they might oscillate superficially faster than the underlying futures. All things combined, the edge in the narrow spread from the ETN may not be that large as we have perceived let alone the fact that you can place a trade at settlement for the VIX futures. That is only my guess since I have not really traded both instruments for the same strategy to see which one is better.