they're as safe as any other key.
There are other potential safety issues, so I'd create them with vanity gen and offline.
Thank you for your reply!
As mentioned above, they may not be safe at all. Hard to know since they were created by an unknown tool. But yes many users have been robbed of many bitcoins because they used tools that use low entropy or other implementation errors. Especially do NOTdo things that increase the danger with such keys like re-using the addresses, accepting several payments to the same address. If you're concerned about these keys, and since you don't know how they were generated I think you have good reason to be, just sweep the funds and be done with them.
