1 There cannot be a "LEase" in perpetuity.
2. But since, ur vendor has acuired the said property under "the lease in perpetuity" and if his possession has been undisturbed by the legal representatives of his vendors for a period of 12 years and if his possession is undisturbed by the govt for a period of 30 years, then as per law,notwithstanding any other acquistion or disturbance in his title or otherwise, he has then acqiured valid title to convey the property to u.
3. But, since these kind o f documetns give room to arguments that it is a lease and not an absolute sale, Ask ur vendor to trace out the Legal represenattives of his vendors i.e those claimants under the executor of the lease deed to, Settle or convey the prpoerty to u under a separate registered deed, in order to safeguard ur intrests