Yes, this is a known vulnerability. Indeed, several alts have been tanked in this manner. However, it was done with several times the previous hashrate, not a fraction.
The reason this could be done, is because 100s even maybe 1000s of times the equipment for that hashrate existed outside of those altcoins. Even before bitcoin was on ASICs it would have been extremely difficult to round up as many GPUs as bitcoiners were using at the time, and that was a device that had general purposes, like gaming, and was being used for scientific computation in some larger supercomputers.
So, there does not exist a significantly large amount of SHA256 hash power outside of the bitcoin network, there's a percentage on some SHA256 Altcoins, there's some old avalons that suck 10 times in electricity what they make in bitcoin, you could buy up every single one of those, blackout a decent sized city trying to power them up, and probably not even be able to pick them out from the noise/luck variation on the network hash graph.
