A VCO will proiduce a note continuously. You need to use an ADSR (Attack, Decay, Sustain, Release) attached to the gate output of the MIDI node and connected to a level node. You can then use the level node to control the output of the VCO. Something like this:
By adjusting the controls on the ADSR you can control how the volume of the note changes with time. The Attack and Decay happen at the beginning of the note, Sustain controls how loud the note is while you hold down the key and Release controls how long it takes for the note to fade away after the key is released. Have a look ate the tutorial by @RobertSyrett: https://discourse.audulus.com/t/so-you-just-got-audulus-what-now/201?u=stschoen