VTRM stands for Virtual Table Rights Management.
On PS3, VTRM is responsible for BluRay DRMs. (need to merge infos from PS3 wiki).
On PSVita, VTRM is a partition on the internal eMMC, with the short name tm0:. It embeds files about activation: NPDRM, Marlin, Kit activations files.
On PS4, VTRM embeds information about NPDRM act.dat. And maybe more (need to merge infos from PS4 wiki)
On PS5, maybe VTRM has been renamed to FTTRM.