How do you control the short VIX future potential risk over nights? If something like 9/11 happens in weekend then there is nothing you can do about it and VIX future can raise like 300%, 500% or even more.
That is why I mentioned trading them outright is too risky and is basically a directional bet. Switches/Calendar spreads and even FLYS have better margin and can be modeled for less risk exposure to overnight BOOM or even normal vol spikes.