If your PLC's already serve as OPC Servers and you are looking for ways to get the data into your PI System then you can use OSIsoft OPC interfaces (not part of vCampus). But if the PLC's don't already serve as OPC Servers, I am not sure here is the best place to get your question answered.
Another option might be using other off-the-shelf OSIsoft interfaces to grab data out of your PLC. PI OPC DA/HDA Servers could then act as OPC Server. To see if there is a suitable interface for your specific PLC you can contact OSIsoft Technical Support.
As Ahmad has pointed out techsupport is you best bet. However, seeing that I know the Siemens controllers really well I thought I would throw in my comments. Normally we use TCP/IP.
For the S7-400 I would recommend purchasing a CP443-1 card and Simatic Net. This will provide you with an OPC connection to the PLC. For the S7-300 firstly be aware that older 300's support only a limited number of connections; we actually avoid the 300's. You could use a CP5611 or CP5614 card which I believe come with an OPC server. The other alternative is to get a MPI interface card from Siemens (I can't remember the product codes) and Simatic NET to connect to the controllers. One thing to bear in mind is that you can daisy chain Profibus or MPI connections so you don't need multiple cards.
There is one sutble problem with using MPI; there is typically only one per controller so if you are using it for data collection you will have a problem connecting a PG to the PLC.