Is it possible to add a 3.5 mini lcd display to use a secondary display for the vim series boards?
Of course. But how does your question relate to the Tone Board?
The connectors are on the tone board
The OLED I2C and the I2S connectors of the TB are just extensions from the large VIM 40-pin connector. Nothing more and nothing less. They just pass-through the signal lines from the VIM board connector to the TB connectors, if you put the TB on the top of the VIM. So, there is no difference except that you need to use the different (and less amateur-friendly) connector on top of the TB.
Oh ok thanks for the information. I didn’t know