Difference between port scanners and vulnerability scanner

Port Scanners and Vulnerability Scanner

Port Scanners and Vulnerability scanners are somewhat similar in function while there are some difference in them.

Port Scanner

Port Scanners are the network tools used for scanning a range of IP Address to check for port availability, operating System Versions used and running services in a host. While these have some disadvantages as NIDS logs and firewall Logs can easily detect them because of the significant amount of the network activity. 

The best port scanner tool used today is nmap because it is free and easy to use.

Vulnerability Scanners

These are a bit advanced form of port scanners. They act like port scanners by looking a range of IP address while the only difference is that they check for known vulnerabilities. For an instance, if a v 2.2 of program named x has a vulnerability, then if hots uses same version then it will warn for vulnerability. So this tool is helpful to maintain network security.
Popular Vulnerability tool used today is nessus.
The only difference between nessus and nmap is that nessus is difficult to use and configure while both are freeware.