The only reason free hosting exists at all is that it is easy for a host to attach it to ways of earning back what it costs to provide and make enough money doing so to cover the expenses of it.
To date there is no such method for offering VPSs, so the best you can hope for on a free VPS is having a generous host sponsor one.
Problem with sponsorships is they rarely last. All too often a host will sponsor a VPS and then either run into financial problems theselves, or end up needing those resources back for paying clients and abruptly cancel it.
I don't think you're going to do much better than the three providers Sander mentioned, all three of which are rather shifty.