You must have a really small portfolio. That is the only way anyone would even consider shorting 140% of their portfolio with VXX. Must be less than $5000. If you have a $10,000 or more portfolio, then you better have a home equity line of credit to back you up if something unexpectedly rocks the markets.
If your portfolio is worth $50,000 to $250,000, then you are playing Russian roulette. If worth more than $250,000, then you are just plain stupid, and should have someone else manage your financial affairs!
Just because you can't do it doesn't mean other people can't do it. You don't strike me as being all that savvy a trader - yesterday you said you didn't even know why your broker limited your buying power. Really?