simulating protocol is easy if it is already implemented in simulator. if it is the case then you just need to call agent function for that. I did it in ns2 for tcp-vegas. May be you can search for the tcp-vegas implementation and add it to opnet then simulate it. But here you need to double check that the implementation is correct. A quick google gave me this link:(attached)

http://en.pudn.com/downloads384/sourcecode/internet/tcp_ip/detail1650107_en.html