The other community members have given clear advice. I would also like to add this: Don't trust VM to protect your personal data. Once you leave, whether within the 90 days or after that, the email can be hacked and your personal details used for fraud, your friends and family spammed. (Well to be honest, plenty of people get hacked while still active customers, but at least they can regain control as paying customers!)
You can get started now choosing a new email provider, plenty of free ones, and some you can pay for. They may have better security, better support and good spam filtering.
And I always counsel to backup important emails from the VM mailbox to your own secure location. When you have got most things transferred to your new provider, you can set up an autoforward in webmail to help pick up anything you have missed.
Turn off all spam filtering so you don't have to log in again to retrieve mis-identified spam. Don't use the holiday notice - that could alert spammers and scammers to your new address. Delete all emails and contacts from webmail. This reduces the data that could be lost if anyone hacks in. When the 90 days are up run the address throughhttps://mailtester.com and make sure it is removed from VM's server. If still showing valid, come back here and ask them to delete it manually.