I'm wary of this as well, which is one of the reasons I'm hesitant to ever make a guarantee to users unless I have insurance for all the funds. Maybe BitVPS.com can convince us otherwise?
Instead of managing users' fund, just use your money. For example, if you could only risk 1 BTC, than only keep a cold wallet deposit of only 1 BTC. That way if the VPS get robbed, you lose your 1 BTC instead of the customers' money.