In searching for information for physical COM Ports vs USB to Serial in VM's I have found a very mixed variety of answers.
I have a couple USB to Serial adaptors and have very mixed results with the drivers and how or even if they are presented to the VM.
Some single port adaptors drivers do not let VM's see them as physical so they will not work correctly in the VM.
Some only show up in VM settings removable devices as USB controllers, what a head ach. Don't move it, 9 out of 10 times it will fail then when it works it disconnects and reconnects and the VM will not install the driver for it. Way to many hours spent on that one 
On the good side, I have found that the SIIG USB to 4 serial port driver DOES appear as physical ports to VMWare if it's plugged in before you start VMWare. 
I have dug through many post in the community and some had just enough information to get you to try things but not enough to make it work.
Here is what I have found. Confirmation and additional information posted here in one conversation will help me and many others as I can see I'm not the only one looking.
This is a comment I left on someones post:
I think if VMWare sees the COM and LPT ports as physical devices in the (add>serial port>use physical) then those values can be used in the serial parameter section in the vmx file and serial0 and serial1 would be what the virtual BIOS uses for its 2 serial ports. And what about thin print taking or using a port, how does that fit in if I need all 4 ports, turn it off??? I have to make VM's out of some old equipment that will only support old OS's because of the software that can not be upgraded so I need to use the serial and parallel ports as physical devices in these VM's, work a rounds will NOT do it.
Also, where can I find a complete .vmx serialx parameter list?
Thank you for reading.