Ensure board is always on the same port?
Normally, the COM port number wont change, but you cannot simply rely on this.

Depends on various effects, especially if you have multiple USB-Serial devices, update the driver, etc. Also, there's the possibility to clean the COM port numbers in the operating system to get rid of unused legacy entries, which will restart counting them from fresh.

Most applications search for all existing ones, eventually display their names, and leave it to the user to choose the desired port.

And, btw. it's not related to VB.net