Generally need to select COM2 in the VM
Not sure if this is your issue, but be aware that VM generally uses COM1 for printer redirects by default, so depending on how you are mapping your port into the VM, you may need to select COM2 in the VM.
When it comes to serial comms and VM's, I've generally have better luck using a physical serial port...most laptops don't have them, but you might want to check if the laptop brand you are using has a legacy port adapter. I use Dell E-series Latitude and the legacy port adapter Dell sells for those is a fairly slim module that connects to the docking port slot and has a 9 pin serial and 25 pin parallel port on it. No need for USB to serial, which I know can give issues. When using that, you add a Serial Port into the VM's hardware config and tell it to use the physical port...this is where it gets mapped as COM2 into the host (You'll notice when you add the hardware it shows up as Serial Port 2 in the VM hardware config dialog).