There are only a very few of the Vectrex games that actually took advantage of the analog joystick. So playing Vectrex games with a digital joystick works perfectly fine.
Regarding the buttons it varies a lot from game to game how many of them that are actually used. Most games don't use all 4 of them.
Some examples:
Mine storm: Escape, Thrust, Fire (3 buttons)
Scramble: Laser, bomb, laser, bomb (2 buttons doubled)
Berzerk: Fire, fire, fire, fire (1 button)
Hyper chase: Up shift, down shift, gas, break (4 buttons)