The VT500 series were the last terminals produced but Digital Equipment Corporation. There were 3 models. The VT510 monochrome single session, the VT520 monochrome multi-sesson and the VT525 color multi-session. They were introduced in 1993 (VT510) and 1994 (VT525). DEC then sold its terminal line to Boundless Technologies in 1995. DEC itself was sold to Compaq in 1998 and Compaq was acquired by HP in 2002. Boundless Technologies moved the production back to the US from Taiwan. So a quick way to distinguish the two is by country of origin. After DEC disappeared totally, Boundless Technologies kept making them but instead called them ADDS VT500 series. And they are still available new! Perhaps the last DEC product being manufactured. There are derivative products from the other lines, but as far as I know this is the only unchanged product line.
I have one VT510 and two VT525s. The VT510 has the monitor incorporated into the terminal like a tradition terminal. It does differ from the previous DEC terminals in using a standard PS2 keyboard. On the rear, you have com1 is on both a male and female DB25, com2 on a MMJ and a parallel printer port (yet another nod to PCs) on a female DB25. For the communications you could pick either com1 or com2 with either com2 or the parallel port as your printer.
My VT510 was made in 1993 by DEC and it is now the terminal I use the most. It was the terminal I used on the PDP 11/23 and PDP 11/73 at the beginning of NaBloPoMo 2009.
The VT525 is a multi-session color terminal. It takes the use of PC components one step farther. It uses a PC monitor in addition to the PS2 keyboard. It supports 3 sessions. It has the com 1 on db25s like the VT510 along with two com ports on MMJs and a parallel printer port. This one was made by Boundless for DEC in 1997.