Bilateral Persistent Fetal Vascular Syndrome (PFVS) is very uncommon in my experience. Moreover, the diagnosis is very suspect because there are other conditions that really look quite similar.
I will make a bit of a fine distinction. The fetal vessels feed the developing fetal eye but when it matures to near the final form these vessels are no longer needed and the fetal vascular involution or atrophy is part of normal retinal vascular development. In fact, many conditions that disrupt the vascular development also disrupt the involution of the fetal vascular system. In distinction PFVS is a non-involution of the fetal vasculature in the face of otherwise normal vascularization. It is a local disease of the eye and not of the person.
These other disease are often bilateral therefore the bilateral persistent fetal vasculature is mostly a secondary result of another disease of vascular development. Most common would be Retinopathy of Prematurity (ROP), Familial Exudative Vitreoretinopathy (FEVR) and Norrie Disease. I have also seen it in bilateral combined optic nerve and retinal coloboma. These cases are not PFVS but FEVR (for example) with persistent fetal vasculature.
The diagnosis can be differentiated in many cases with the peripheral angiogram or sometimes from genetic testing (however, the diagnosis rate of the blood test is still pretty low and is not much evidence against the presence of the clinical diagnosis)