is it possible to protect workstation by removing VCC connection from workstation to circuit?
No, it still will not protect against overvoltage on the serial ports. There are a few ways to do this here are two suggestions:

You still need to run the numbers on the current and size the resistor correctly. If the laptop port needs 1mA @ 5V to recognize a high or '1' condition, then you'd need 5V/0.001=5k max resistance, but you would also limit the current to the port and prevent a burn out. Diodes can also be used to clamp the voltage on the board to prevent the voltage getting higher or lower than VCC and ground. 

You could also use a zener diode to keep the voltage below a certian value, in the other example, the zener will turn on if the voltage goes beyond 5V and shunt excessive current. 

There are other ways these are just a few suggestions. Make sure your ground is connected to the laptops ground or you might have issues. If the VCC of your circuit is not of the potential of the port on the laptop (you haven't specified where you'll connect it with the laptop, then don't connect it. )







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