The very first thing to try is to access your email via the webmail interface. Open a web browser and go to
Login with your normal email username (your full email address) and password, and see what is there. If the missing emails are seen then it is probably an issue with views or filters on the iPhone or iPad. But if (and I’m afraid based on your description, I fear this may be the case), the missing email are not there, then I am afraid they are gone permanently.
I wouldn’t want to speculate as to what may have caused their deletion, but VM do not keep backups of email, that is entirely the customer’s responsibility. The webmail interface is a view of what VM currently have on their servers, if the email isn’t there then, I’m afraid it is gone.