VTxx refers to V.R. Technology's "NES-on-a-chip" Famiclones, some of which having greatly extended capabilities. These capabilities are described in separate articles:
The progression of features as well as backwards-compatibility is contiguous until the VT09. The VT32 and VT369 are the lower- and higher-end enhancements to the VT09, are not compatible with each other, but are backwards-compatible to the VT09 except in the format of the enhanced palette and the absence of the second APU.
V.R.T.'s website has datasheets for VT16- and VT18 chips that are merely VT09s with additional LCD controllers.
The VT1682 and VT268 are not NES-compatible, even as their designs are inspired by it.
CPU $0000-$07FF: 2 KiB of internal RAM
CPU $0800-$0FFF: VT01-VT03: Mirror of CPU $0000-$07FF, VT09+: Further 2 KiB of internal RAM
CPU $1000-$1FFF: VT01-VT32: Mirror of CPU $0000-$0FFF, VT369: Embedded 4 KiB ROM
CPU $2000-$2007: RP2C02-compatible PPU registers
CPU $2010-$201F: VT02+: New PPU registers
CPU $2020-$207F: VT369: Newer PPU registers
CPU $3000-$3FFF: VT369: Mirror of PPU $2000-$2FFF, allowing direct nametable access
CPU $4000-$401F: RP2A03-compatible APU, DMA and I/O registers
CPU $4020-$403F: VT02+: New APU, DMA and I/O registers
CPU $4100-$41FF: VT02+: New miscellaneous registers
CPU $4800-$4FFF: VT369: 2 KiB of RAM shared with sound CPU
CPU $5000-$53FF: VT369: Mirror of PPU $3C00-$3FFF, allowing direct palette access
CPU $6000-$7FFF: Optional WRAM, if present on cartridge, VT369: Optional ROM bank
CPU $8000-$FFFF (write): VT02+: When forwarding is enabled (register $410B bit 3 "FWEN" =0):
MMC3-compatible registers that are forwarded to $4101-$4108)
CPU $8000-$FFFF (read): Four 8 KiB PRG-ROM banks.
CPU $0000-$07FF: 2 KiB of scratch RAM CPU $1800-$1FFF: 2 KiB of RAM shared with main CPU CPU $2000-$2FFF: Sound registers CPU $4000-$4FFF: Embedded 4 KiB ROM
PPU $0000-$1FFF: CHR-ROM, all modes: CHR pattern data, bit planes 0 and 1
PPU $2000-$2FFF: NTRAM (2 KiB, selectable mirroring)
PPU $3C00-$3EFF: VT369: CGRAM in 8bpp modes
PPU $3F00-$3F1F: CGRAM in 2bpp modes, entries $3F10/$3F14/$3F18/$3F01 mirrors of $3F00/$3F04/$3F08/$3F0C.
VT03+: Also CGRAM in 4bpp modes, bits 0 to 5, of colors 00-1F; same mirroring of address bit 4 applies.
PPU $3F20-$3F7F: VT03+: CGRAM in 4bpp modes, bits 0 to 5, of colors 20-7F; no mirroring of address bit 4.
PPU $3F80-$3FFF: VT03+: CGRAM in 4bpp modes, bits 6 to 11, of colors 00-7F; no mirroring of address bit 4.
PPU $4000-$5FFF: CHR-ROM, 4bpp modes: CHR pattern data, bit planes 2 and 3 (VT03+)
The VT01 is V.R. Technology's first Famiclone. Its only enhancement is the ability to drive an STN display directly using a modified palette.
The first of the V.R. Technology Famiclones to add major enhancements:
The VT03 greatly enhances the graphical capabilities of the original RP2C02:
Low-cost replacement for VT03.
Current revision of the VT09.
Final enhancement of the VT09.