How can I make an acoustic guitar VST sustain its notes in Cubase 12?

[SF_Green]

I think Lillie has you covered:

[Solved] Export Just One Instrument From A Score As MIDI File

It should be - in Setup mode, move a layout that only contains the instrument you want to export to the top of the Layouts list. MIDI export uses the top layout in that list, whether it’s a full score or a part. (Exporting audio uses the top-most full score layout, including if that’s at the bottom of all the part layouts.)