I'll have my opinion on that post, but don't let it affect the decision.
"Modularizing" is indeed a good thing to do, it may also help users who're looking to improve the code/learn from it as an example to know where the should be looking. It also, as you said, shortens the main file.
I'd rename vertsms.py to vertsms_gui.py though (I think fMMS follows that way - and I like it best)
Importing modules when they're needed also reduces the resources needed to start the app.
It wouldn't affect packaging, some minor changes to the .desktop file, py_compilefiles currently runs on all *.py file so again it shouldn't matter.

Again, just my opinion