Ken's right, this is gonna be a tough assignment if you aren't familiar with plcs.
How's your vb? What version vb?
You mentioned RS232, and this will work but won't be ideal for mass data polling. Are you talking vb6 or .net? Quite a bit of difference in rs232 comm coding. Again, the plc type will dictate everything.
If this is straightforward data entry, I think your best option is to skip vb and use Excel and DDE as Ken mentioned.
Good luck!
-Mike