Vector is wonderful, and while I have no experience with Loki, if it's as bad as this post and thread suggest, perhaps Vector makes it more manageable by normalising and buffering everything coming into it.
For example, another comment here talks about Loki locking up Docker if it's the logging backend and the container crashes. I suspect that wouldn't be possible, or would be less likely and more manageable with Vector in the middle because it will buffer. I've also dealt with normalising logs from different sources and it can be a pain, but Vector will do some or all of that already, reducing the requirements put on Loki.
That doesn't mean much unless you're their customer. "Works well enough and we have people that know it" is perfectly fine way to pick a tool, even if it is not technically the best one.
For as much functionality as Systemd replaced (when you think about it it's an extremely ambitious project), it has been an extremely smooth transition.
For example, another comment here talks about Loki locking up Docker if it's the logging backend and the container crashes. I suspect that wouldn't be possible, or would be less likely and more manageable with Vector in the middle because it will buffer. I've also dealt with normalising logs from different sources and it can be a pain, but Vector will do some or all of that already, reducing the requirements put on Loki.