They can't be at the beginning which sucks, they have to be in the middle, but nonetheless you can have a vanity string that is 12+ digits long contained in an XMR address with only a few minutes spent checking addresses. And yes, you will have the private keys, I am not talking about burn addresses. Here are 2 that I have personally made, one took 6 minutes and the other took 6 hours:

47Uo6EFMW5FBEosp951V9SdiiyZLkATdafCu2ofxtremegamer1wYkchuK1R6sW4mp6bTFQANzf2gbtidM4yoCD6EoJXphP
465sL8j2WqWZu7i7B7t9iKYyMbUpAvi9tgNPMDjMommyMi1kers96hEYQTimtvNa56cN7tjVqDxUKXMQDbzikXcJ199V8cu

Longer ones are definitely possible, you could hit 13 or 14 if you have a few hours, and if you have days 16 definitely isn't out of the question. As you can see they are a bit more difficult to parse than vanity addresses with the vanity portion at the beginning, but I think the sheer length of these vanity strings can make up for it. If you want to know how to do this you should read my blog post here: https://github.com/xtremegamer1/How-I-generated-the-longest-ever-xmr-vanity-address.  Does anyone have thoughts about this?