Create a shortcut. Right click it and go to properties. In the Target field, append -console to the end outside of the quotes.

"C:\Program Files (x86)\Activision\Vampire - Bloodlines\vampire.exe" -console

In the Library tab, right click the game and go to properties. Click set launch options… and add -console then click OK.

The console should be visible upon startup of the game. It can be hidden/shown by pressing the tilde or `/~ key. Sometimes the console can glitch a bit and can only be closed by pressing the tilde key, or by clicking the x in the top right corner. It can also freeze the game, but can usually be unfrozen by opening and closing the console again.

To issue a command, enable the console and type the basic format:

<command or variable> <value>

so for example..

cl_showpos 1