For programming vcodes, I’d like to refer you to Section 10.7 VCodes – Programming Visual Stimuli of our Creating Task Programs for Dexterit-E 3.7 guide. If you have any questions about the material there, I’d be happy to answer them.
To toggle a target on/off, you need to change the second element (ie. vcode(2)). So you could have a Stateflow state or MATLAB embedded function that outputs a 1 for one seconds and 0 otherwise to vcode(2).