particularly para 6.5 - makes it quite clear that that it is YOUR responsibility to back up the contents of your email account.
Whilst VM does have recovery procedures for data loss arising from hardware and software failures at total Email Service level, VM has never offered a recovery service for individual user emails which have been accidentally or deliberately deleted.
No major email provider will guarantee to recover lost or deleted individual email data. In a recovery situation their focus is to be able to recover the service following any major failure, from the system backups. The policy of various email providers about individual email data recovery varies from - “if it’s not in Trash it’s gone” - to “we will also look in some of our backup disc storage which we recycle every 7 days approx”.
Gmail - “When you delete a message, it stays in your Trash for 30 days. After that time, it will be permanently deleted from your account and can't be recovered.”
Yahoo - “If any of your emails have been deleted or gone missing in the last 7 days, you can submit a restore request and we’ll do everything we can to try to recover your lost messages.”
Microsoft Outlook.com - “When you accidentally delete an email message from your Outlook on the web email, you can recover those items if they're still in your Deleted Items folder, and in some cases, you can even recover items after the Deleted Items folder is emptied.”
VM’s policy is similar to Gmail – but the Trash seems to be emptied at variable intervals.
Over the years we have had a steady trickle of cases which have very similar symptoms to yours – namely emails which have mysteriously disappeared. The vast majority of these cases involve the use of Apple devices (iphones/ipads) and/or email clients (such as Outlook) all connected to VM Webmail and therefore each other via IMAP such that a single delete command on any one of these devices is replicated across all these synchronised devices.
Apple is adamant that all such incidents are attributable to “user error” of one sort or another and not to bugs in the apps. Various popular descriptions include - Fat finger syndrome, Bi-focal hit and miss, Doing email on the commuter train, etc.
The other main possibility in your case is that your email account has been hacked and the hacker has tried to cover his tracks by deleting the contents of ALL your email folders - and failed to do so. As a precaution against this you should action the advice given here -