I know of two ways of using VPN, one being through remote desktop and another via settings you can enter on your device which changes how you connect to the internet, it will go through your ISPs servers and then to the VPN host the out to its destination.
Whether your external IP will be the same or not every time you connect will be up to the VPN host, but usually you'll have the same IP for the length of one connection at least, then when you disconnect after 30 secs it may change depending on if there settings have set a static IP or dynamic IP, static means it will never change and dynamic means it will but the the frequency isn't always the same but it can be, again its dependent on the hosts settings.
If you was accessing two different email accounts and send mail from them on the same device, the likely hood of them being prefixed with the same IP is quite high, unless the VPN host has set dynamic IPs and yours has changed since you sent the last email, which is very slim nd your device needs to be disconnected for at least 30 secs defaultly, however it can take longer. And by disconnected in all the paragraphs I litterly mean thedevice is not connected to the VPN in anyway, even if your device is connected but your not doing anything on it, its still sending data and reviving it, ie telling a server to keep a.connection open.