To expand on what Harvey suggested, first be sure to have the
Tool Options Bar visible, to get that go to the menu
Windows > Tool Option Bar, then select your Brush Tool (I assume you can already see the Toolbar, if not do the same procedure as before)
When using the Brush tool, you can access the Accuracy feature to make the line correction better or worse on a Vector Level. Note the "Smoothing" value range is inverted in OpenToonz as opposed to a progam like Flash.
So, If you give it a value of 1, you'll get complete line "auto-correction", which is not what you want. But if you give the Accuracy a value of 100, the smoothing algorithm will be practically turned "off" (the smoothing is so minimal you won't notice).