Suppose I am using public WiFi and connected to my office network through VPN. I am accessing some applications which are not SSL enabled. How easy it is for an attacker to see the traffic if he is also using the same public WiFi.
If the concern is just as far as the WiFi hotspot and the users who are connected to it, then as long as you are connected to the VPN, the traffic should be encrypted and cannot be viewed by any other WiFi user.
There are some concerns however, on the type of VPN used, which encryption method it is using, and so on. Also, I think the biggest concern would be if the VPN gets disconnected without you knowing, which would automatically fall back to unencrypted WiFi traffic.
Depending on your OS, you could configure it to stop all traffic if VPN is not connected at any point of time.
It is as strong as the SSL configuration of your tunnel between your laptop, and the VPN gateway.
All traffic now goes through the tunnel to the VPN gateway where it reaches out to the Internet. The VPN gateway will then ferry the response back to you.