I have just done my Vito, take off low pressure pipe to reservoir, connect a metre or so of clear tube, oh have a drip tray under car and as you remove the hose from the reservoir replace it with a bent over bit of rubber pipe held together with a zip lock to stop fluid escaping. Jack front of car up enough to have wheels off the ground.
Have a helper to turn the steering slowly from lock to lock, the fluid will start to come out the clear pipe, as it does so top the reservoir up, don't let the fluid get to low before topping up, I used a funnel with a piece of tube on, the diameter size of the reservoir hole.
Remember you do not won't to get air into the system, you should see the fluid colour change, when you do, you know you have replaced it completely.
Reattach the low pressure hose, again turn the steering lock to lock just to clear any air you will see inside the reservoir any bubbles, if air is present. Top to correct level and that's that. Keep everything spotlessly clean, clean around the reservoir before and after, you do not won't any grit or whatever in the system. Can mess up your rack.
You must use the correct fluid spec for your car, pick your moment and get 60%off eurocar parts or gsf, or of course Porsche.
Please note I am not a mechanic, this is just what I did, personally if there's nothing wrong leave well alone, sounds like it's more to do with the pipe fittings, I would check those.