Some VST plug-ins have different timbres based upon the velocity. This is why different velocities will sound different. If you wish to always use the same velocity, you will want to set the velocity option in the inspector, per the instructions in this question:
How do you make Studio One ignore incoming MIDI note velocity?
If the velocity option is not showing up for you, you will want to click and drag the handle below it, until it shows up. It may just be hidden.
If you don't want to send velocity at all, you will want to contact the manufacturer of your keyboard for information on disabling touch-sensitive keys.