I'm curious about this as well. Where's the value in trading these against each other?
We do trade the VXX against the futures, but only as an extra venue for added liquidity when hedging our options trades. You can find the VXX formula, but it is adjusted daily as they roll the futures. There was a time when the etf would fluctuate maybe 10 or even 20 cents through "value", but I haven't seen enough edge in that trade to make up for slippage and trading costs in at least 9 months.
So an OCT VIX future vs VXX position is a cost and haircut heavy way of replicating an OCT/NOV vix future spread.