You could use the new One Touch Switch (OTS) process with Plusnet
https://www.plus.net/broadband/discover/one-touch-switching/
to switch from VM to Plusnet on a chosen day.
If you want to overlap the two services (to ensure continuity of service) you would get your new Plusnet service installed and overlap with your existing VM service. Once Plusnet is up and running to your satisfaction, you give VM 30 days notice to cancel services when your VM minimum term expires (to avoid early disconnection fees).
OTS should switch you on a particular date but runs the risk of no service if some part of the process goes wrong. With OTS, Plusnet should deal with VM so you don't have to (although a few topics on here have mentioned a few issues with OTS as it is still a new service).
Overlapping allows you to try out the new Plusnet service in your 14 day cooling off period and have VM still running as a backup if Plusnet turns out to be no good or a delay in installing Plusnet crops up. The disadvantage of overlapping services is you will pay for two services for the overlap.
If you want to port a landline number you should be able to incorporate this to OTS but if you overlap services you need to make sure your existing landline number is in working order with no disconnections pending on it for Plusnet to port it over.